Allison is crap for anything over stock and waaay too expensive to build. Go with a built 48re if you want one to last. Manuals don't like to survive at high hp, especially with twins making a ton of power down low.

 

If your going to swap any way why not put something like a 9 speed eaton in it? It can be done and it would be a blast to drive! I'm not an Allison fan, or any other auto, for that matter. I always wanted to put more gears in one!

 

they all have their limitations. Allison got the GM contract because they were low bid. they are a good trans, until you start building the big power. then they (like most others) fail. with the hp numbers you're quoting, you need a built trans- auto or manual. the weak link in either case should be tires. they will lose traction long before a good trans is overtaxed. My '03 is pushing 550 hp and 1100 ft. lbs, with a stock 6 speed. only issue is the clutch- just not enough to hold the power.