Chevy Engine Swap from 8.1 to ???

[trinic]

My brother owns a 2001 HD 2500 (non-4x4) with the 8.1 Liter engine, which is attached
to the Allison Trans. He asked me to look around to see if it were possible to
replace/swap the engine out of it with something more economical....basically, something
that doesn't SUCK-UP so much GAS! From my point of view, he probably won't gain much
out of it. He currently has about an 6-8 inch lift, 30+ tires, big heavy A** bumpers, etc.....
Either way, I'd like to give him a formal answer with a bit more knowledge than I have. If it's
possible and someone has more knowledge of it, please enlighten me a bit. What engine
size/year do you recommend? He also plans to leave the Allison trans in the truck, so he'd
need to find an engine that will mate to it without any extensive mods. Will the computer need
to be replaced?
Thanks up front...