My recommendation -- Forget the 400-hp figure, and skip the rear-end swap for now. Use your stock 5.3 cam, spend a couple hundred on a complete gasket kit for the engine, and assemble it as is. Bolt it to the nv3500, and stick it in the truck. Go with a stock ECM, and just do a mail-order delete on the VATS, emissions, and auto trans. This should get you on the road for the cheapest possible.
400-hp would be nice, but a stock 5.3 at slightly over 300, is
far better than what you currently have.
When you find a better rear-end, swap that as a separate project.
Down the road a bit, swap the cam, install a turbo, learn hptuners, and do your own tuning.
I plugged your numbers(guessed on some of it) into a
gear ratio calculator. It looks like the nv3500 should function well enough with a 3.07 rear and 29" tires. Meaning, the shift points aren't wildly out of whack, etc. You may still want to swap the rear end at some point, but I would definitely leave that till later, when you find a good deal, and the budget is a little less strained.