My suggestion would be go to your nearest GM dealership. Get to know the parts manager and the service manager.
GM has some great deals on Crate motors, of all shapes and sizes.
talk to them, tell them what you are gonna be doing with the truck. I am not sure the warranty extends to commercial use, but you can check.
I would be willing to bet a 350 HO or a 383 stroker will fit the bill. of course there is alaway the ZZ4