My suggestion, get a good "racing" or "towing" rebuild kit with upgraded clutches in it and upgrade the input shaft (should run you about $200) and a good shift kit. That is all you'll need to get a TH400 to hold up to most HP levels. The TH400 is strong, you don't need to upgrade any other hard parts until you're near 900HP.

For a shift kit, try to find one that converts the tranny to full manual operation. Depending on your view, you'll either love it or hate it (upshifting and downshifting in every gear like a manual) but these kits put full line pressure into each gear. This is your best bet for saving the clutches since there won't be any slippage.

For the parts, check out Dacco-Detroit. They are in between Himes and Dale Mabry near Westshore. They sell to the public, but they ONLY deal in auto tranny parts and do carry high performance stuff.