LS in the TA. Save the original motor and trans. You could most likely do it without hurting any future collector value. Keep a manual trans. A 5-speed would most likely fit without hurting anything. The question is, do you really want a "collector car" that you're not inclined to drive for fear of hurting it's value or do you want a hot rod? Those 70's era engines were not known for big HP numbers. Is that what you would be happy with? What would give you the most day to day enjoyment?

Vortec in the Camaro. The 400HP you want should be doable with it condidering how good the Vortec heads are.

FWIW - I sold a nice straight 55 2-door post because it was original and I decided I didn't want to take it apart. That was a decision that I have regretted ever since. I'll never find another one that was that straight and rust free for the kind of money that I paid for it, but it was a 6 cylinder/OD car that I would never have driven much.