Boosted LS1

Ok, I've decided an auto is what I want for my turbo project. I've heard the 350 may be weak as may the 4L60E. I'm wondering about 400's or 4L80's. So if I said I wanted a trans to handle 800 lbs/ft or more what would I need to consider? I find a trans brake very appealing and that seems a possibility with a 4L80. I've never seen an american auto in front of me but a t56 just fits the tunnel of my car width and height wise. I may have access to a GM350 in the near future, is it similar in dimensions to the boxes I've mentioned?

Sorry for all the questions but a few brief pointers will help a lot. Thanks.

Mike.