8.8" Rear end to support 600 horses

Your best bet would be to build the 8.8 if you want to have it handle that amount of HP.

I had mine built to handle alotta abuse :

31-spline axles
C-clip eliminators
T/A cover
Auburn Pro
3.73 's

see many people run high 10's to low 12's with this set-up.I will say the weak link that some people have in this set-up is the Auburn Pro as once it wears out,it's no good as it cannot be rebuilt,so other then that,it's a pretty solid combination.In your case,I might even consider getting axles with more splines yet.

If you can find a 9inch set-up I would get that if you dont feel like building the 8.8