No AFM nor DOD on the hummers

 

I'd agree with tharber. Put a roots supercharger (Eaton makes the actual innards but Magnuson, and Edelbrock both come to mind for brands) on it. That type makes its power across the entire RPM range and is instant while the turbo style supercharger and turbos have lag and are more upper end of the RPM useful. Also put an intercooler if at all possible and full long tube headers. Get a custom tune done at a shop that known what they are doing. I only did full new exhaust (long tube headers + stainless Borla cat-back), Airaid intake tube (the stock airbox is already a top notch cold air intake and filters far better than any of the cone garbage), and a custom tune. Made a massive difference....

 

the original 6.2l is running what 403hp and 417tq?
the b8 is a nice engine but you could just make the difference up with a supercharger on your current 6.2L
if you are admant on upgrading engine i would get a little bigger on the engine, not sure if we can fit anything bigger in here but LSA, lsx454, ls427 all look like good bets because then you are already pushing enoigh power stock and then you can always add a supercharger in the future if you really wanted to but that would be hard on the tranny for sure

it all depends on your budget, probably best to run 91 octane and get a super charger kinda thing