727 Rebuild!

Most stock parts are well up to the task of handling 400 - 500 horsepower with ease. The high performance clutches and steels are nice insurance though. However what ever else needs to be replaced will have to be determined after teardown and thorough inspection. Chances are though that if it's working good now it won't need much.

As for the valve bodies, well, if you have the money but lack the expertise to modify one then they are a bargain and in some cases are better than even an experienced rebuilder could do as some have purpose built transfer plates that are beyond the scope of the average hobbyist. It's really up to you, a really good valve body can be built with something like a Trans Go reprogramming kit, but it's a bit involved, or you can just buy on from Turbo Action, TCI, ATI, or any of a dozen other shops.