Moulder,
whatever yoy do, do NOT get a 53 block, mine cracked and I replaced it with a 01 mexican block. The block that has a bunch of small numbers just below where the inj. pump mounts is the mexican... I just had it done and it works great.
When my block cracked, I tried the Lock N stitch repair... didn't work... another guy did it and made a new crack 3" from the old one so stay away from the 53...
I would say your twins and box and such setup might make 400 - 500 rwhp... with no nitrous. . Water/meth would be a good thing as it lowers EGT's
I second the vote for a 6 speed manual unless you modify the Heck out of an auto with tripple lock and such.
The G56 on the new trucks uses the NV271 transfercase and that would be best with a new 3rd gen truck block. but yeah, build the heck out of the inside with that crank!!