Good to see you here 2lane, don't know your level of experience or ability, also there are some tools required that are only used for trans work, do you have this level of equipment? There are more than a few manuals / books to guide you, Google TH400 overhaul and see the results.

It isn't terribly hard to tear down a Turbo 400. But reassembly with out damaging the seals and o-rings takes some practice and knowing the "feel" and that's where experience comes in.

I don't want to discourage you from doing this, just want you to be honest with yourself about your abilities.