[455firebird1969]

After doing a little web research, it seems all Allison transmissions behind the Duramax engine, have the chebbie bolt pattern. This would indicate a 2006 or newer Allison transmission might be a good option. I'd rather swap the rear differential, the but that might not be as easy as it sounds.

[pfilean]

If the Allison Chevy bolt pattern is the same as the 350/400 bolt pattern then there are several adaptors available. Either with or without transmission mounted starters. Of course Bruce's comment about the 1/4 inch difference might require some research to pick the right adaptor. But if you used a 61 or later block (64 and later have block mounted starter) it would still look about the same as the 347 in the truck. Hardly anyone except a real Pontiac person can recognize the reverse flow cooling anyway.

[Bill Hanlon]

Originally Posted by pfilean

But if you used a 61 or later block (64 and later have block mounted starter) it would still look about the same as the 347 in the truck. Hardly anyone except a real Pontiac person can recognize the reverse flow cooling anyway.

You'd have to do a bunch of motor/tranny mount work as the early engine had 1 mount in the front and 2 mounts on the bell housing.

[455firebird1969]

The Poncho engine will be a 64 and later with a block mounted starter. At first I was thinking a 455 would be best, but I'm starting to lean towards a 400, or even a 350 two barrel. I guess I need to research rear ends that will bolt in. I don't think I need the added weight and low ratio of the one in it.

[Jack Gifford]

Originally Posted by 455firebird1969

... I was thinking a 455 would be best...

In my opinion a 455 would be best for such a large truck. The 455HO (now 467) in my Suburban is very pleasant (and fun) to drive. Weighing about 5,000 lb., the fuel economy is only 14 MPG but- it also got 14 towing a car home from Oklahoma! And it's happy on 87 octane.
Glad to see you "saving" that GMC. Are you on the oldgmctrucks forums?

[455firebird1969]

Originally Posted by Jack Gifford

In my opinion a 455 would be best for such a large truck. The 455HO (now 467) in my Suburban is very pleasant (and fun) to drive. Weighing about 5,000 lb., the fuel economy is only 14 MPG but- it also got 14 towing a car home from Oklahoma! And it's happy on 87 octane.
Glad to see you "saving" that GMC. Are you on the oldgmctrucks forums?

I've been watching stove bolt forums. As far as this truck saving me, it just kinda happened...I saw it and I had to have it. It has more character than anything I've seen for a while. It picked me, so to speak.