For your consideration, this is the combo I'm planning on using on my next project (3804, the 1 ton pickup).
1. Swap out bellhousing crossmember for a 54 or 55.1 crossmember that has the angled mounts.
2. Use a 55-72 v8 or 63-72 inline 6 bellhousing (whichever years had the angled mounts on the bellhousing). These have the standard gm transmission bolt pattern on the trans side, and the standard sbc engine bolt pattern
3. NV4500 transmission behind the bell (I already have an adapter plate made to bolt it to the bellhousing). You might not need to worry about this part if you want to keep your five speed since it probably has the standard bolt pattern that would fit on the later bellhousing
4. Cummins Chevy breadvan adapter (these are clocked 5 or 6 degrees, so your engine will look crooked) and a 4bt.
For a long time I thought using a 4bt was a waste of time, but then I drove a truck with one. It was a '54 1/2 ton, and it was a very good engine for that truck. The motor was coupled to a 700r4 (first mistake), and it had too big of a turbo on it (most of the time it wasn't spooled up because it was so big, but just running the truck on the motor provided plenty of power to move the truck).
The truck had the original frame, and none of the front sheet metal had been touched, so it will fit in the truck well. The guy who put it together had a custom front motor mount made that even used the stock front motor mount location on the 54 frame as well.
The bad: the truck vibrated a lot at idle, and the truck was noisy. I'm not sure how the vibration will play out in my truck. I've read you can bump the idle up, and that will take care of most of it. Once the truck was moving (the 54 I drove) the vibration did go away. My truck has always been kind of noisy, so I'm not too worried about the noise. I just use earplug on long trips.
I used to think the power would be too low because looking at the stock numbers it makes it look like a 261 would out perform it in everything except fuel mileage (10-15mpg vs 25-30mpg); however, you can always turn up a stock engine to make more power. I had thought that turning it up would drastically reduce the life of the motor, but a little digging over at 4btswaps will show that there are a lot of guys who have doubled their power output and have had reliable motors that don't blow up.
After seeing all of that, I'm sold on it. It will fit. It will have more than enough power. And I have an overdrive trans that will help with highway speeds. If the project ends up being too uncomfortable (vibration and noise), I'll just pull the motor, sell it (4bt won't loose any value provided you didn't get ripped off in the original purchase), and swap in a 292 (I never liked the idea of swapping in a v8 because of the fabrication needed since I like to keep the frame, suspension, steering, and bodywork stock where possible).
That's what I would do, though if you wanted to keep sparkplugs, a 261 or 292 coupled to an nv4500 is what I would suggest
