I see you have the Bjorn / Buick rim details going...

1. Do a search about tires, there have been endless threads about this.
2. Ditto
3. Used, here or Epay. I don't think there is a repop of these.
4. Ames, Classic, NPD, some others. Classic has rediculous shipping prices, but they are in CA, so maybe not so bad for you. Try classicindustries.com.
5. Hood tachs were an option, not stock for the 400 model. These can be found used, or repop. Rear spoilers are for Camaro's, not Firebirds. (ditto for front spoiler)
6. Many options out there. Most through Epay or repro houses. Stock front sway bars seem small, but overall stock suspension leaves a lot to be desired anyway.

My biggest question to you: Why not take everything back to stock, rather than continue a modded car? Looks like a clean starting point.
Also, order PHS to verify originality and build info.

Nice car!