I think T56's can handle more power than they are rated at but I still believe that if you have good traction your 1000 horespower would destroy a stock t56. I have one and love it, but I only have around 400 horse. Here are some links for t56 stuff. You should be all right with a t56 that is modified for the power. You might want to consider a gear vendors under/over with your th400, or a 4l80e and paddle shift.
http://www.g-forcetransmissions.com/default.htm
http://www.ddperformance.com/
http://www.t56rebuilds.com/index.html