The Stovebolt Site Forums The Shop Area Engine Shop Heavy Truck Eng/Tranny upgrade

I've spent most of my time (& money) playing with middle 50's Ford F100's. I gave up that hobby. Now I'm looking to keep out from under foot around the house in retirement & need something to play with. A 58 GMC 450 4x4 caught my eye. Aftermarket Howe conversion. Not ever having done a rebuild on a heavy truck, updating the Engine & Transmission stumps me. It currently has what the former owner says is a 302 straight six going through a 5 speed with hi/lo range transfer case. 1st question- what will bolt up to that transmission? I'd like to put in a 350V8 ( 400 small block would be better but finding one...) Then the idea hit that at my age, automatic would be nice. I have the 350v8 setting out back still bolted to the TH350 transmission but would that get the truck rolling with a load in hi/street range of the transfer case? the 6 cylinder in the truck currently does not start. Former owner a young guy & I'll bet never seen points before but when I bought the truck I calculated in for bad motor just in case.... Any one have any suggestions? Being 4x4 with plow & I'm up in ther mountains I would like to have it as a driveway backup plow truck. To that end I'd rather not go down the diesel road again....

I'm not up on that particular swap, but it sounds like a great truck to keep original. The 6-302 5-Speed sounds like a strong combination to me. I'm not much of a fan of an automatic in any truck, but a TH350 in a 4 X 4 along with a plow pushing snow is asking for a lot in my opinion.

I agree with C10-C90 Bill,I would work with what you have now.The 302 has plenty of guts to get r done.

F100Panel,

I recently obtained a 55 GMC 350 COE and I think the engine and transmission (4-speed) are OK but would consider a higher speed (lower ratio) rear-end transplant. It now has the stock Eaton 2-speed rear end (its a 5.83-8.10 or 6.33-8.81).

Your engine/transmission should be strong enough (but maybe you need/want the power/oomph of the 350?).

Tim

I have been down that road. I converted a 55 Chev Cameo half ton to a 4x4. I had already swapped the 396 into the truck ( 1970-71), and what I did was buy a 76 Blazer and swapped in the TH350, NP 203 transfer case, front, and rear, differentials. I also retained my stand alone overdrive ( Hon-o-drive). I did have to modify the driveshaft. I welded a new plate on the front and rear ends to match the 55 springs.
It is a lot of work, and now I am going to tear it all out and go back to 2wd when I install the 454 TH400.
I agree with C10-C90 Bill that the 302 is a good engine and you can make some serious power with it. If the running gear is good, why change it, especially if it is one of the factory authorized conversions?
Bob

Welcome to the 'bolt F100!

That 450 is going to be a heavy truck, so with a 350 you're asking a motor that likes to make its power turning fast to work hard going slow. And the T350 is not a heavy duty transmission. I think if you want to use this truck as a truck, this is the wrong way to go.

But then I saw a '41 or '46 GMC 2-ton at the ATHS show in Shelton last week that had a 350 and an automatic with otherwise original running gear. The impression the owner gave me was that it is servicable if the demands put on it are like a 1/2-ton pickup.

The 450 series is a 3-ton truck, so this is heavy stuff you are dealing with.

I would take a look at the school busses on eBay for a possible donor if your want to go V8 automatic. If I remember correctly, they used either a 366 or 427 with an Allison? automatic which would work much better in a truck that large. School busses seem to have low mileage and go pretty cheap. If the body is aluminum, you might be able to sell it for more than the price of the bus.

Got exactly what I was looking for here/THANKS FOLKS!!!!!!!!!!!!! I have a 74 International AWD Loadstar with 17K miles that I was going to restore until under the factory double frame I found a section of the mainframe rusted out. If I can't get scap value for the truck/$600 I may pull the engine & transmission ( 345/5 speed) & look that over as a possible swap.. Those 345's were nice... The automobile transmission swap had me doubtful though now I'm SURE its a bad idea.My neighbor has a tired 3 yard payloader with Detroit power & when the transmission on that went he put a 350 Chevy with TH350 in it & has been using it for 3 years now & it works as good as ever. That was what got me thinking/looking down a bad road.... The Allison truck transmission with matched engine idea needs some thought too/THANKYOU... I'm sure I'll be back with more questions. This heavy truck/GMC world is all new to me....

OH!!! I forgot to mention, I was looking at Chevy 350 due to the wide array of fair priced aftermarket upgrades. One of wich is a rockcrawler cam. Brings the maximum torque range band on the 350 doiwn into the hi idle to 2500 rpm range. I believe Edlebrock supplies those...

Kenny Bell, one of the hi performance transmission builders from the mid 70's, says that a built up TH 350 is only good for about 900 hp and put his transmissions in many championship cars to prove his point. To say that the TH350 is a weak link is too simplistic. They can be modified to take a terrific amount of punishment. The TH 400 is even stronger.
Bob

I forgot to add to the above post that a buddy of mine ran a very hot 350 engine with a B&M blower driving a TH350 in a 40 passenger school bus converted to a motor home for years. He had no problems with the transmission, and I can guarantee you, that as a retired top fuel driver, he beat the crap out of it. Synthetic transmission fluid and plenty of cooling helped keep it working right.
Bob

Well, since you are going to scrap that 302 anyway... I CALL DIBBS! :p

Hey F100Panel, I would try to adapt a Turbo 400 Trans w/od and rebuild that tough ole' 302 inline six. If you balance it and put a manifold for a 4bbl ( 600 cfm Holley ) or 2 deuces it will have more torque than any sbc. I have two 350s and they run super smooth and strong. In fact I'm selling one in a bout a week ( I'm the second owner ) as I am putting in a modified 250 six with an offy manifold and Holley carb. Those 302s were originally built for the military. Its not a 270 bored though based on that engine. I would keep that motor and put MSD on it with a few mods and you can plow snow as long as you want. Small blocks don't put their top torque on the bottom end like an inline six does, especially one that large. I would have the block decked like mine and the head milled about .030-.040. They took .030 off mine. A 203 transfer case or one from a military deuce and a half will work just fine. SBC are everywhere and common but 302 inlines aren't and are nostalgic and built like an IH rig. BTW, that 345 will outpull a stock 350 ( yawn ), anytime. They have high nickel heads and block which makes them very hard. The only weakness was the lifters which I had no trouble with, but excessive wear was due to lack of changing the oil. With sythetics no problem. Taken care of they last forever. There are special cams, manifold for the 345 and other parts available for them. Do the same tricks to them as any other like what I mentioned and you will not be disapointed. Some porting and polishing and etc. Some were bolted to a 727 Torqueflite trans from Chrysler. A good tranny. Had 12 IH rigs and love them. Can't wait to be stovebolting here real soon.

NP205 is far superior to the NP203. Course the new Atlas 4 speed is even better, but big $'s.

The 302 will give you that cool nostalgia engine. Lots of torque, but no high revs, and parts are expensive. You can install a 454 in that engine bay, with some "enlarging" of the firewall and "modifications" to headers. I don't know if a tall deck 427 will fit. SBC will fit no problem.

400 tranny is better than 350. How much torque you gonna put through it?

Are you certain that the original engine is trashed? That 302 will make gobs of torque, which is what you really want in a 4x4, unless you're building a monster truck. Adapting a V8 shouldn't pose too much of a problem, you'll need to find a Chevy V8 bellhousing with the same trans (probly an SAE #2 pattern) or possibly a GMC V8 bellhousing, which would be a BOP engine pattern. The transfer case should be a divorced system, so only the shaft from the trans to the 'case may need alteration to match whatever trans you choose. Personally, if you were to go automatic, I'd suggest either the TH475 or some sort of Allison, like from the aforementioned school bus. Don't worry about the T-case strength. Most likely it's either a Rockwell or Timken unit, or maybe a Coleman. Either way, they'll be much stonger than any later model T-case.

Having thought over all the above I believe I'll put the effort into getting the 302 engine running. The truck was ordered new by the Sykorski (I'll bet I killed the spelling on that..) Helicopter Company to plow their Airport. I do not know if it was Factory 4x4 or an aftermarket. It has a Coleman Center load front axel.... Sykorsky kept it till around 1980. After a short stint hauling firewood it whent into storage. The engine spins over just won't catch. The vehicle will hit my property Saturday & shortly after that I should be able to try to get her running. Mean time, everyone offers that the engine is a beefy torque producer. Does anyone know off hand the HP/Torque specs just out of curiousity?

Most likely, it's an aftermarket, and probly also has a Coleman T-case, too. Probly a model 22 part time, although there was also a model 42 full time case also. I do know that GM did use Coleman parts in the mid-'60s on heavy duty 4x4s, however I'm not too sure about any that were built factory before that using the Colman parts.
When you get her home, can you put up some pix? I'd really like to see it!

I've sent pic's & a little background to the photo Gallery on this 1958 AWD GMC 450 Dump. Should be able to see'em in the gallery in a couple weeks. THANKS for all the information the group has supplied so far. I'm SURE I'll have more questions!!!!1

Nice to know that another potential V8 swapper has seen the light and will keep the old 302 going. It has more power than you think and can make tons of power if worked over.

Good call.