I'm no transmission builder, so I'll just relay what my trans guy has told me and what's been done to my 727. It's held up to 1100+ hp in a nearly 2 ton car and doing fine.

5 pinion steel planetary...b/c the stock aluminum ones will strip the splines right out.
Bolt in super sprag.
A&A billet steel front drum.
Heavy duty front band strut...the stock ones bend.
TA pro tree transbrake and a deep cast aluminum pan.
As far as I know, everything else is pretty much stock. I know the input and output shafts are factory pieces.