I'm planning on using a Duramax/Allison combination in mine (to keep the truck all Chevy).
Really, what about Isuzu? Not to give you a hard time, well not much of one,

just couldn't let that pass...
It's the thought that counts right?
While "brand loyalty" is something, I don't think it should come before reliability and practicality. If you are essentially building a truck from scratch It's a mistake to overlook potentially suitable parts because of who you think made them.
I am building my own truck, and I don't care who made the parts so long as they perform flawlessly and provide the simplest and cleanest solution to the problem at hand. My engine, while it says Detroit Diesel, was actually made in Brazil. My Brake Master cylinder is from a Ford, because it was the simplest to make fit a remote reservoir, motor mounts, ford again, because they worked the best of any I could find. I also have plenty of GM stuff, but only because I figure they are the best parts for each individual job.
Grigg