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I am at the beginning of my 3600 restoration project that includes plans to convert to a SBC and I am looking for input/feedback regarding the approach others have taken. Overall project goal is to have a great looking truck that I can drive and show that will be comfortable to drive on country roads at 55/60 mph. May do some limited hauling/towing but not really a priority.

First some basic thoughts (nothing set in stone):
I plan to keep the 3/4 ton 8 bolt wheel pattern and original axles/suspension, I have read a bit about rear axle ratio changes to reduce the original 4.56 ratio to 4.10 and am anticipating going to a NV3500 or 4500 transmission to get overdrive, I am anticipating a disk brake conversion for the front and possibly the rear, I anticipate converting the steering to power assist, and I have a 350 SBC that spun a rod bearing so needs a complete rebuild. Not sure what wheel/tire size to shoot for but clearly that interacts with the rear axle ratio discussion.

I am considering building the SBC as a 383 given that I need to replace all the rotating parts anyway but realize either a 350 or the 383 will have a lot more power than really needed. The engine was from a TBI version, 1991 Chevy 2500 but I anticipate utilizing a carburetor set-up rather than fuel injection to keep it simple. I do not anticipate using the 1991 cylinder heads due to poor performance so probably Vortec heads with the correct intake manifold to utilize a carburetor. Cam selection to be matched to the displacement/heads/compression ratio etc. to provide a reasonable idle and low end torque appropriate for a truck engine.

I have seen several options for SBC engine mounting (bolt in and weld in) but I am a little unclear about the need to modify the firewall for bell housing clearance and/or rear engine mount changes required. The original in-line 6 has a 4 speed SM 420 transmission which just hung off the back of the bell housing but I needed to remove the transmission separate from the engine because the transmission would not fit between the cab and the rear cross brace. Ideally I would not modify the firewall for clearance but as I need to make cab floor repairs as part of the project I would prefer to know if modifications are needed before I get too far with the cab floor repairs. Also curious about drive shafts as the original was a torque tube from the transmission to the intermediate bearing with conventional from the intermediate bearing to the rear axle. I have read somewhere that a single shaft from the transmission to the rea axle may be too long.

I have read about exhaust manifold to steering gear clearance issues with various solutions including: no problem with original gear, spacing out the original gear with a modified drag link, newer gear (manual or power) mounted outside the frame and forward of the axle, and probably a few others but curious of pros and cons for various options given that I prefer to go to power steering anyway.

I anticipate utilizing a new radiator with either an engine fan/shroud or an electric fan/shroud set up.

I have also read about fuel tank options for under the bed near the rear with the fill either located in the bed floor or possibly out the side. My first thought is to go out the side but would appreciate input from others regarding what tank and fill set-up was chosen and any pros/cons.

Thanks for reading.

If you are using the original frame and leaf springs, I recommend that you stick to a 350 in its original configuration without modifications.
These trucks are not nimble regardless of disc brakes. Emergency stops combined with avoiding contact with obstacles is not the strongest suit of these trucks.

WICruiser First done a SBC in 1967 in a 53 3100 older hot rod pals said you couldn't do it. Don't have to bother the firewall,have the thickness of your palm between the corner of a tin valve cover and the firewall. Next use a 66big car manifold on the steering side,shim steering box out 5/8 inch. end up with your carb mount -up level. stay centered in the frame.used the short water pump radiator in stock location'was a 265 bored to 283 2 2121s carter AFB's 098 cam.worked good !!

52 Carl your right ,don't handle like a stingray !!

The first thing you mentioned was the rear end gear. Using a 410 rear end gear is going to get burn a lot of gas. A buddy of mine is running a 370 gear with a 350 Chevy trans and is running 3500 rpms at 60 mph. You might want consider a 350 gear with an overdrive trans to lower your rpms. If you don’t modify the firewall and you run an HEI distributor it will be close to your firewall. I just modified my 46 firewall because my distributor was too close. Good luck with your truck.

There are no new 4.10 gears available for the original rear axle- - - -the supply of OEM gears dried up decades ago from us dirt track racers grabbing every set we could find. A late 1960's through mid-70's 3/4 ton Suburban with an automatic transmission, and a few of the pickups with automatics ran the 4.10's that people are searching for now, and those rear ends are very scarce. A better choice of a complete 8 bolt rear axle assembly with better gear ratio selections available would be a Dana Type 60 from a 3/4 ton GMC, Ford, or Dodge box van. The pickup rear ends are a bit too wide to fit your truck without fender modifications.
Jerry

With 5.13 gears and 32.5 inch tires my 51 turns 3300 at 60. 2800 in overdrive (17%). Running empty my warmed over 235 could easily pull a 4.10 axle. Not so, loaded to the gunnels as it usually is. I'm following along because I've considered eventually installing a SBC so I don't have to shift gears so much.

Ton_tommy, thanks for your information, my 4.56 gears (original) would run about 10% lower rpms and if I find 4.10 gears it would run another 10% lower. What size tires are you running that provides the 32.5" diameter?

I have read some about converting the rear axle to something more current and the overall width being a problem but how much wider are they and what fender modifications are typically done to accommodate them?

I did a similar project a few years back to put a SBC400 in a 1992 Jeep Wrangler. I built the engine with a short water pump and had a home made shroud with electric fan that had good clearance at the front of the engine. Long story about the 400 selection but I understand the firewall clearance concerns for the distributor, etc. just wasn't sure about bell housing clearance as I have read that the firewall may need to be cut out for clearance.

Fixite7 - did you modify or replace the rear engine mount? My impression is the original rear mount needs to be removed and a new rear mount installed in the proper location for the transmission mount.

I have a 3600 running 30” tires, I’m looking to go to a 3.73 FD to raise my cruise speed. You can swap to a 14bolt for pre 85 trucks pretty easy. The hub to hob with will be the same 72” bit the wheel mounting surface differs between axle versions. You’d want to find one with no wider than a 67” wms-wms. Then any fine tuning to tuck the tires can be done with wheel back spacing.

A SF 14 bolt has 6 lugs, is an option for half ton trucks, FF 14 bolt from c and c truck is narrower.

Ed

If I understand the terminology correctly my rear end appears to be a 10 bolt fully floating (FF), picture attached. I have not cleaned it up and checked for numbers so I cannot confirm if it the original. It appears to have input shaft seal leak.

Pinion seal.

How many bolts hold the wheels on?

Ed

8 bolts hold the wheels on, sorry the attached doesn't show it well but I think you can tell the axle is a full floating version given the axle attachment.

Having second thoughts about the engine and rather than building a 350/383 I may go the LS route. Biggest question is how to tune the electronics of the LS given this truck will be nothing like anything an LS would come from and I would still prefer a manual transmission. I guess I need to do some more research.

7045 george Forgot about HEI clearance on my advice to WICruiser,need to adress that,hear WICruiser ??? I used NAPA gold points to achieve high RPM with small distributor.

fixite7,
Thanks, I understand the firewall to distributor clearance issue in determination of the engine location if I go with the Gen 1 SBC. If I go the LS route I believe the location constraints are similar but different.

Looking like a strong possibility that I am going to go with a 5.3 LS engine and 4L60E transmission from a 2004 Silverado 2WD. Assuming I can get the engine, transmission, ECU, transmission controller, and all the related wiring harness everything should talk to each other just fine. The 4L60E has a lower numerical final ratio in overdrive so depending on tire size selected I can probably run the original 4.57 rear axle and have reasonable engine RPM at 55/60 mph.

The only drawback to that rear end is that it's extremely heavy. We used them extensively on dirt track race cars back in the 1970's and 80's, and they had no problem handling 600 HP or so and 7K+ RPM on a rough surface like a rutted clay track. The only drawback was getting enough shock absorber action to keep all that unsprung weight under control. It's too bad we also scavenged up all the 4.10 gears we could find and wore them out in the process. A Muncie 4 speed transmission with a 1.88:1 second gear running against a 4.10 was just right for a "long quarter"- - - -a track that measured 1/4 mile around the infield. A "short quarter" that was measured around the outside wall needed a 4.56 to keep the RPM right.
Jerry

So as an FYI, there is no transmission controller in that setup. The ECU controls both the engine and the transmission. If you are transplanting the whole thing like that, all you will *need* for tuning is a VATS disable. You might consider also getting the rear O2 sensors turned off at the same time. There are a few extra considerations you might want to be aware of:

A 2004 could have a cable drive throttle or an electric throttle. If it has the electric (Drive by wire or DBW) throttle body, you will need the matching pedal and throttle body controller. The advantage to the DBW setup is that you get cruise control for free. The disadvantage is that you have to deal with the electronic pedal and the controller for it. You can swap from manual to DBW (or vice versa), but that requires a segment swap in the ECU.

A 2004 should have no EGR and no air pump.

A 2004 could have a mechanical fan or an electric fan. The electric fan is controlled by a ground switched relay. If it has the electric fan, you can just mount a mechanical fan on the water pump and ignore the fan output. The ECU is not smart enough to notice the fan missing as long as the engine is not overheating.

The LS expects 2 cats and 4 O2 sensors. The front pair of O2 sensors are used for closed loop control and really should be installed. The cats and rear O2s can be left off, but if they are not disabled in the tune it will set the check engine light.

There is a lot of good information here: https://www.lt1swap.com/wiringharness.htm about modifying the wiring harness on an LS engine to run standalone. It looks daunting at first, but just take things one step at a time and it is actually pretty simple. You will end up with a bunch of connectors for the engine/trans, 2 connectors for the ECU, a fuel pump connector, +batt power, +ign power, and ground.

The potential 2004 disappeared so I am back to trying to decide what I want and balancing that against what I can find. The good news is that given the body restoration etc. part of the project is going to take a while I can procrastinate on my drivetrain decision for a while.

One thing I have been thinking about is given that the truck is a 3600 and I am trying to stay true to the heavy-duty nature of it compared to the 3100 I wonder if I should be looking for 4L80E (if I go automatic) and possibly a 6.0 engine. I am sure it would be overkill from a pure performance perspective but the drivetrain from a 2500 HD 2WD with the 6.0 and 4L80E would "fit" the 3600.

I am fairly familiar with the general sensor/wiring requirements for the ECU controlled engines but appreciate the link to the wiring information. I really like the benefits of EFI but have enough background in carburetors to feel comfortable going that route for the simplicity (relatively speaking).

If you do go for a 6.0, check the shape of the transmission pan before you buy it. Some came with the 4L80e and some came with the 4L65e. Most of the 4L65e setups were in the Cadillacs, but not all.

You can swap transmissions, but it requires both a segment swap in the ECU and some wiring changes. The guy that runs LT1Swap has a $75 ECU programming service that I have never heard any complaints about.

OP, you asked about tire size for a 32.5 tire a couple of weeks ago. I'm running 255/85X16 which is about the same diameter as the original 7.50X17. I believe a 245/70 X 19.5 is in the same ballpark.

WICruiser I would bolt a piece of 3/8 x4 x10 on the trans mount then center and let it slide on the crossmember till you get clearance for your HEI ( I forgot that didn't have it in 67) that picture of the 47 suburban with PVC question shows the firewall/ valve cover clearance wider than palm thickness because of HEI I think. you'll have to estimate on engine weight to keep carb close to level,can help it some back at the steel plate. Welded my side mounts out of 3/16 plate framed-up hollow on side mounts, could use 55 chevy type at front but seems kinda top heavy with a torquer and it's a long span especially with aluminum in between there. 1/8 would be plenty on those mounts at the side.

WICruiser On your steering I used the pitman arm and draglink with no modification just shimmed the steering box out 5/8 of an inch. Have heard of shimmy if draglink isn't paralell with road surface,never heard of that till stovebolt 50 years later. Took great pains to be sure axles were paralell and perpendicular to the frame,used a 57 car rear end with traction bars about 4 ft long. Used 54 car hubs on front to match wheels.

WICruiser post lost again to cloud

WICruiser Put 3/8x4x10 onthe trns mount let it slide on the cross member till you get your engine situated fore and aft,keep your carb mount level.

fixite7, thanks for the input. I have a lot of decisions to make before I am ready to locate the engine/transmission (such as what engine and transmission to use) but I will be referring back to all the input received as I proceed on this project.

The "edit" button at the lower right of the page allows you to add information to a post without starting a brand new one.

Like this

And this

And this

And one more time- - - - -like this!
Jerry

Beating a dead horse.

Looks like I am heading down the SBC 350/383 engine route with an NV3500 manual transmission.

I am planning to build the engine with 350 Vortec cylinder heads so working through the pros and cons of going to a 383 displacement given the increase in compression ratio. I have a fair amount of work to do before I need to make the decision although engine part lead times are pretty long these days.

My cylinder block is being bored oversize (not sure if it will be 0.030, 0.040, or 0.060 over until it gets machined).

I have been researching cylinder head options assuming that I go with the 383 rotating assembly and wanting to keep the compression ratio around 9:1 so that I can run regular gasoline. If I used dished pistons (roughly 18cc) with a chamber volume of 64 cc I am in the ballpark. The two head options I am looking at are Vortec 350 heads or just using the original TBI (1991 193 casting) heads. I know the TBI heads are considered boat anchors by most in the Hi-Pro world but given that the goal is low end torque and not high rpm power the fact that original heads drop off over 4500 rpm may not be a big deal. Of course the other advantage is that I already have them, although they need to be cleaned up and checked for cracks I believe they are a viable option. I have seen options for Vortec #906 casting heads with prices all over the map. From what I have read the Vortec heads would flow much better at higher rpms and therefore provide more peak horsepower but I am not sure they would provide any higher or broader torque curve.

Either way I anticipate needing to buy a new intake manifold to match the head choice and provide for carburetor mounting.

Any insight regarding the head choice would be appreciated.

I am also looking into cam selection and would appreciate any insight members could provide.

Vortec heads take a different intake manifold than the earlier heads.

If your focus is on torque and not top end I would use your current heads. The money you save will let you have a good valve job done and also help with a good cam selection. Your cam is where you need to do your research, a low end torque cam will make all the difference.

Thanks for the insight.

I understand the intake manifold difference but anticipate I would change intake regardless so the only question is which version I would obtain.

I have submitted a tech request to Comp Cams and hope to get some insight regarding the cam selection from them.

I will have my machine shop check out/refurbish my existing heads and head down that path. Worst case scenario is that down the road I need to change heads at the added cost of gaskets and the different intake manifold.

A 383 using the 76 CC "smog" heads from the mid 1980's should come out with a static compression ratio somewhere in the 9:1 range or so. With a smooth combustion chamber that doesn't have a lot of hot spots, that's near the upper end of 'regular gas" compression. The effective compression pressure will depend on the cam grind, with a more radical cam resulting in a lower effective pressure due to the intake valve remaining open for part of the compression stroke. I have had very good luck with the Comp 268H cam in a warmed-over street engine. It pulls strong on a dyno run all the way to 7K RPM or so, with pretty good midrange torque. The idle is slightly choppy, but not enough to require a high idle speed.
Jerry

Thanks for the information on the Comp 268H cam, it sounds interesting.

I believe the heads (either original or Vortec have 64-65cc combustion chambers combined with the 18cc dished pistons and some assumptions about deck height, head gasket volume, etc. I calculate a static compression ratio of 9.3 but as you indicate the dynamic compression ratio will depend on the cam. I may be pushing the limit for regular gas but I will know more when I can confirm deck height, cam selection, etc.

WICruiser Did I come back??

WICruiser To answer your question,used side mounts with a plate off of tail housing mount. Felt like front mounts would make engine assembly top-heavy,was iron bell housing.

I am anticipating using one of the SBC engine mounting kits for the engine mounts (haven't decided if independent or tube type), anticipating needing to remove the original frame cross member that provided the rear engine/transmission mount, and using one of the after market transmission cross member mounts located at the transmission mount location (determined by the engine location). The NV3500 has an integral aluminum bell housing. As a result the engine and transmission assembly should mounting should be similar to '90s Chevy truck structurally.

My understanding is that at least some of the SBC engine mounting kits have dimensions from fixed frame locations to position the engine for required cab clearance. I suspect my cab may be off the frame at that stage of the build process so I would appreciate any insight members have about the resulting engine to cab clearance. I may need to drop the cab back on the frame just for peace of mind when I have the engine mounting mocked up.

WICruiser, Just a suggestion, measure and mark the location of of your firewall in conjunction to your frame. Use a straight edge held vertically on your firewall and mark where it intersects with the frame on both right and left side. Use a know reference point on the frame, a front shock mount or radiator core support mount to take all your measurements from, some point that will not change.

If you can mock up the engine and trans prior to making any permanent mounting points you will be happier. Finding an interference point during muck up is easier to adjust the once you have everything bolted on welded in place (don't ask my why I know).

The NV3500 should be a good choice for your setup. The shifter location is just about right for a 49' and with the right cam and rear gears it will work great. Do Your Homework!