9 1/4" Axle Swap?
1) The 9.25 axle is a heavy pig. Not suitable for car use IMHO. That being said, I do see lots of 400+hp Rams and Daks at the strip, and they rarely have axle problems. Having rear disc brakes solves the potential flying tire issue.
2) The 8.25 axle can be reasonably built to stand up to 400+hp no problem, and they are cheap/easy to come by.