As an owner of a LLY myself, I quite like the water methanol...
You WONT see 75 HP.
The best I have heard of is 30HP on the Dyno. My D-max probably see's 15 horses outa the water methanol. The best thing about it is EGT control. With EFI live you will be able to turn up your fueling with out worrying about EGT's going through the roof. You can also advance your timing a bit farther then others not running the methanol. Ramp up the boost too on that VVT!
check out this link for install on an lly.
05 Chevy Duramax LLY