What I meant by going away from GM power, is that I'm going Cummins, donor truck came with a BB 454 V8 gasser in front of a three-speed - and got just a hair over 7MPG on the 300 mile recovery trip. My 40' Gillig transit bus got 8MPG on it's recovery from Oregon, it weighs in over 20klbs empty and mostly dry, and has the aerodynamics of a small house. With a properly tuned and geared 6BT, I'm hoping to get more like 20+MPG in a 1-ton crew-cab dually, similar to what I'm getting in my 2-seat 4-banger Toyota pickup now (what I'm replacing).
I think OP is just going for "easy" to get up and running after diesel engine fail, so that means staying GM as a power-plant. I'm just another yahoo that's doing a Cummins in a square-body, so I don't have much to add to his situation. But cucvnut might, since he got my old engine.