If you are planning on the power levels you are talking about, get a good one now instead of tearing the car apart a couple more times. I highly recommend Phoenix Transmissions. They did an unbelivable job on my 4l60/4l65 trans. and their customer service was outstanding.
http://www.phoenixtrans.com/html/gmtrans.html
Prices listed here are for new transmissions, but they do have a rebuild/upgrade service that I am sure is cheaper.
-Nick