Honestly, at that HP level, you are basically asking a race car to perform street duty. You are going to have to expect something to break at one time or another...that's the nature of these things, especially turbocharged cars. Believe me, I've had my share of them....and they're toys, pure & simple. I always had a backup daily driver so it wasn't a big deal until I had to call for a tow home.
The best bet is to keep the manual transmission if you MUST drive it and race it, but don't expect things to last forever. As the HP goes up, reliability goes down squared. Experience speaking of course.
At 1000 HP and turbocharged, a powerglide would fit the bill perfectly. But, like mentioned they don't like street driving because you're constantly on the converter and that makes a little heat. The C4 would work, but you'd be better off putting clutches in the T56 once in a while. C6 would work too, and probably be the best bet, IMO. How about a 4R100? They're pretty tough as well...but they're comp. controlled and you'll need a controller to run it and they ain't cheap. The TH400 isn't a bad transmission at all...pretty tough...but they eat up power, they're heavy as could be...no OD, no lockup feature (always on the converter=heat) and they have their downfalls. They're a pain to overhaul and build as well. Some folks can make a TH400 work better than a glide, but others just cuss at it and throw it off a bridge (including me).
Thinking about it, I think that the 4R100 would be a great candidate...it's already got the Modular bellhousing so that takes care of the adapter plate issue if you have a Mod motor. Just have to beef it up a little and do some research...but it should work great. BTW, they use the same trans in 2nd gen lightnings and also the F250/F350 super duty pickups.
