400hp won't hurt those trannies. There are guys pushing 700+ through them without much worry. Both of my trucks are 450hp or better on the nv4500 with no issues. I lost 5th gear in the dually, but that has nothing to do with power output, and in the scheme of things, is a minor failure, just have to use 4th. Replaced it with a tranny that has 400k+ km on it and its holding just fine. You are underestimating what the drivetrain in these trucks are capable of.

 

It only gets expensive when you hear the crunch...