Don't forget that you will need bearings, axle shafts, the 3rd member (with differential, bearings, and gears) plus brakes...
Before you commit, recognize that unless you start with a used axle and are lucky enough to get the correct axle length, expect to pay about $1800 in new parts to complete the installation...
I'll have about $1500 in my 8.5 by the time I'm done, but I was lucky to find a one-legger for $800. An 8.5 is not a bargain if you have to pay top dollar to find one, then pay more to replace all the used up guts.
I concur that the nine-inch is the way to go if you honestly think you are pushing 600HP.