'56 gmc V8 engine swap

[ole black truck]

This will be my first post. Working on a '56 GMC 100, long step side with a V8 (Pontiac) and automatic trans (hydra-matic). I am contemplating swapping engine to a 400/455 Pontiac with turbo 400 trans. Very few parts are available for the older Pontiac engine, most mainly AC. I realize when the first domino is pushed, the chain reaction is very un-predictable. All suggestions will be greatly appreciated.

[Valarius_Starchaser]

Cadillac 500 is my vote but not much easier to find parts for but they make such good torque interested to see what others say

[OrrieG]

400/455 Pontiac would be sort of true to the original Pontiac engines used. Around here parts are relatively plentiful because the inboard boat guys use the 455 a lot. Don't know if you would have the same fitment issues (firewall mod and limited to electric fans because of length) as BB chevy swaps.

[tmoble]

You could go to a 455 Pontiac, they will produce massive torque that will allow you to run a highway gear like like 2.73 or 3.08 and have snappy performance around town. Built and tuned properly they are a real torque monster and do reasonably well on gas. The carb and distributor have to be right or go to a EFI deal.

I know a guy that has a bunch of Pontiac 428/455 stuff in a shed, PM me if you're interested in a turn-key Pontiac deal. In this case I'd go with a TH400 and the highway gear. Nice thing about going with a Pontiac build is it'll bolt right in.

Edelbrock has some really decent aluminum heads.

[ole black truck]

tmoble, thanks for your response. This vintage GMC did leave the factory with a Pontiac engine, however this is the early Pontiac engine an it mounts totally different from the later, '61 or '62 years. This is my dilemma, do I wish to go thru all the fabrication it will entail to switch engines. The parts for the early Pontiac engines are most difficult to find. Thanks again for your imput.

[tmoble]

I'd skip the early deal, build a regular 455 out of a 70s big car, They mounted on the same GM crossmember as all the rest of the GM V-8s. Run a tubular crossmember under the side mounts on the engine and one under the back of the TH400. You can run a long or short tailshaft TH400. You can get these crossmembers cheap, many are bolt-in. Not one person in hundreds even at car shows would be able to tell. As soon as they see the Pontiac valve covers on a GMC truck they'll be impressed. Cool deal, you'll be able to run the engine in the stock location since there's already a Pontiac in there.

One of the nice things is that there's new cast iron cast iron headers available for a performance build or if you want to stay stock you can run the stock exhaust manifolds off the car it comes out of. Either way you don't have to deal with PITA tube headers that always leak.

You could even run a 389/400 with a highway type gear.

As you might have guessed, I'm an engine builder. I'm always looking for a build that's something besides a Chevy. For instance, I recently did a 305 Jimmy school bus engine for a guy that has an antique sprint car he wants to run at Bonneville or one of the dry lakes in California. That's really different. He has a Hilborn alcohol injection setup for it. That's past different.

Let me know if you're interested.

[1project2many]

I recently did a 305 Jimmy school bus engine for a guy that has an antique sprint car he wants to run at Bonneville

Cool engine, but how much did that powerplant weigh in at?? There's a lotta iron in that engine.

[dwcsr]

That's to bad, its sad to see a rare truck like that get swapped out but I do understand the dilemma your in, I'd stay Pontiac with a 4L80 so you have an over drive.

[tmoble]

I don't know how much that 305 Jimmy weighed but I do know I had to get another guy to to help lift the head off the bench and onto the block. Plus, welded a 2x2 45 degree brace into the engine stand and made a new top for it. used 2 pieces of pipe/tubing that fit inside each other, made it about 9" long and welded the engine adapter onto that. spreading that load out made it so I can turn a SBC with one hand.

[tmoble]

dw, it is sort of a bummer to see the old powerplant go but those early Pontiacs are a PITA to do much of anything with. 455s are easy and torque monster effect allows to run a really stiff hiway gear. 2.76 is fine and eliminates the roaring along effect without having to do an expensive OD deal.

the 4L60E/4L80E deal is not attractive to me as long as those limited function tranny computers are so expensive. for no more than they do they should be about a $200 deal. ridiculous what these vendors are charging for those things. I'm building a kit version from the megasquirt people now. about $300 but takes several hours to build. there's a vendor offering a tranny version of their sealed box microsquirt setup now. Don't think there's many out there yet but the MS computer is way more capable than needed for a tranny deal. Still need a TPS which is a PITA if not running EFI already.