I recently converted my 1952 GMC 3/4 ton long-bed 3-window pick up truck to a 355 Chevy small block V8. It runs very well and is strong, strong, strong. But I still have one task left before it's road-worthy. So for the benefit of others wanting to make this conversion, I am posting my approach. The reason for this is because I wouldn't consider this a straight-out swap. Absolutely everything seems to require custom-work and modification. Everyone will have a different approach to this, but here are mine.
Mounting the engine:
Speedway Motors has a few different kits for custom mount applications. The one I chose was a solid-type mount on a metal bar that acts as a crossmember, that can be either bolted or welded onto the frame. I chose to bolt onto the frame because I discovered that this bar REALLY needs to be able to swing and move in order to move the engine rearward to mate with the bellhousing. This is a very narrow engine bay, and I think if I welded it, it would have been disasterous.
Transmission mounting:
I chose to use the stock Muncie 4-speed SM420 because it's such a rugged transmission and I really like the supergranny gear. You can move a mountain off its foundation with that 7.06 first gear ratio. There are bellhousings that mate to this transmission, which are cast iron, and are common in salvage yards. I believe they were used on 292 applications well into the 70's. They have a standard Chevy bellhouse bolt pattern except without the top 12:00 bolt, so they work. I had a 1968 Corvette flywheel for a 10.5" clutch sitting around, from when my Vette was a 327. I had a new ring gear put on and had it resurfaced. I bought a brand new stock '68 Vette clutch and a '68 Vette starter motor, and this works well. I took measurements from the engine to the transmission and found it to be exactly the same as it was for the 235 motor. So I reused the cast iron clutch fork and stock-style throw-out bearing. The stock clutch pedal works fine.
Steering & Exhaust:
Basically what I did was to keep the stock steering for now. I didn't want to get a brand new steering set up until I've completed my 4WD conversion, which will change everything anyway. I just wanted to make the stock steering work for now. Actually, someone here on this forum gave me the idea.
If you unbolt the steering box and do some minor sheetmetal clearancing at the firewall, you can move the box about an inch to inch and a quarter outboard. But to hold it there, you have to use longer bolts and shim it over. I "shimmed" it using brass pipe connectors that I had, but you can use anything.
Keep in mind that it's still very tight. I took out each of the 3 bolts on the engine-bay side of the steering box and replaced them with grade 8 bolts that I had shaved down the heads on with a grinding wheel. Even with that and the shimming, I could not really fit anything in there except stock exhaust manifolds. And because I am using a single-pattern Iskendarian cam that requires a good exhaust system, I really wanted headers. So I picked up a set of 1986 Corvette shorty headers and turned them upside down like you would for a twin turbo application, with the collectors turned up. Then I made my own pipes using 2.5" J-bends, mandrel bent and aluminized. To address the heat issues, I wrapped everything in fiberglass header wrap. That is how I avoid burning everything from wires and hoses to the back of my hands. The J-bends go back down in the rear corners of the engine bay.
Underneath the truck, I used all mandrel-bent 2.5" aluminized pipe. I bought a Summit Racing X-pipe crossover kit for about $85 bucks and have 12" leg 90 degree bends going directly into the X-pipe. The kit comes with four 45 degree bends. I used two exiting the X-pipe and going into 24" long Autozone-brand aluminized 2.5" extensions, and then into a pair of Thrush Super-Turbo aluminized mufflers. Exiting the mufflers, I used two 90 degree 6"-leg aluminized 2.5" mandrel bends pointed outwards, and then used the other two 45 degree bends that came with the X-pipe kit to point it back. Then I bought two Heart-throb brand chome slash-cut tail pipes, 18" x 4" and they exit just in front of the rear tires and stick out about 2" beyond the side-steps. They look nice.
Brakes:
This is the last part of the conversion that I'm still working on. The stock brakes won't work because the brake pedal pushes on a lever arm that hits the exhaust on the V8. For this, I am turning back to Speedway Motors for a solution. They sell a swing-arm racing pedal that mounts to the firewall. On the other side of the firewall, you can use either a power brake booster, or just a manual master cylinder. But it has to be a twin, tandem master cylinder to develop enough pressure for the stock drum brakes. You can use a Chrysler-style master cylinder. I am using a manual master cylinder, but I may change my mind later. The pedal has a 7:1 ratio and the master cylinder has a 1-inch bore, which is what Speedway recommends for manual brakes.
Other Considerations:
Firewall: If you use the stock SM420 transmission, it sets the V8 very close to the firewall. You will have to modify the firewall in 3 locations. Where the valve covers go, and where the distributor goes. It might be better to use a non-HEI distributor because you won't have to do so much modification. I used an HEI, which works, but you only if you push the firewall back a good inch.
Alternator: I used my old IROC alternator and bracket, because it's a big, 93 amp alternator and it uses a 3/4" wide ribbed belt which is harder to roll or throw. The alternator is on the passenger side. I plan to reuse my IROC tuned port injection once I buy the $400 stand-alone wiring harness. Why? Because these trucks LOVE torque, and nothing makes better torque than the TPI. But for now, I'm using a Holley carb and dual plane manifold. In the future, I plan to twin-turbocharge, since the headers are already in the right location, there is loads of vertical space under the hood, and the TPI responds well to forced induction.
Radiator: Not wanting to spend $600 bucks on a stock-style radiator, I went with a modern 3-core truck radiator and mounted it right behind the stock truck grill. I had to fabricate the fan shroud using galvanized sheetmetal. I'm using a stock mechanical fan with a fan clutch and a 2.5" spacer, but I will probably eventually convert to twin electric fans, also off the IROC, which were aftermarket and expensive! The radiator is copper/brass, painted black, and that was chosen because you can't see it through the grill, and so the truck looks stock...if you ignore the sewer pipe exhaust sticking out in front of the rear wheels.
Gauges: I'm switching over to Autometer 5" Antique Beige gauges, which fit the holes and are beautiful. So contact me if you want my old gauges for a good deal.
Wheels: I wanted more of a modern ride, so I got rid of the split-rims in favor of stock chrome year-2000 GMC wheels. They're 16" and have Uniroyal 265/75R-16 steel belted all-season truck tires on them. Look great and ride a thousand percent better than the stock "school bus tires".
Rear Diff: I have the stock 4.56 diff, but since I don't want to cruise at 75 mph at 3800 rpm for the rest of my life, I will be switching over to a Ford-style Dana-60 rear diff with 8-bolt lugs that still fit the GMC wheels and has 3.73 limited-slip. That will lower my 75 mph rpm to about 3100, which is doable.
Gas Pedal: The stock one won't work at all. So some kind of a cable-type aftermarket pedal is needed. I bolted the stock pedal to the floor and used it to mount my aftermarket pedal to. The position is much more comfortable for long drives without fatiguing my ankle so much.
This truck looks TOTALLY stock, so it loses none of its appearance to the V8 conversion. Probably not the smartest choice with $4 a gallon gas, but I figure the 16 mpg I was getting with the weak 235 motor was no worse than the 16 mpg I could be getting with a far-stronger V8. And that will improve once I put the TPI fuel injection back on.
This 350 is not a stock IROC motor. It is a 4-bolt main truck 350 from the 70's. It has 10:1 compression, ported and polished heads with 1.94/1.55 valves, and an Isky 270-Megacam with 221/221 degree @ 0.050" lift. In the TPI IROC, it pulled strong to 5000 rpm, so the combo works.
...Bring on the Porsche Cayennes.
Last edited by Hot Rod Dad; 07/06/2008 5:37 PM.
