[goatless]

How rusty where the body meets the frame? You don't want to drive around in something unsound. That being said there is nothing wrong with having a decent driver- that is, a car that isn't perfect but looks pretty damn good to most people. Most of my cars are that way. Shiny on top, but a little grungy on the bottom. It makes them a lot easier to enjoy when you don't have to worry about getting dirt on the bottom of your car!

Dealer A/C- if it works and you use it, I'd keep it. If it doesn't work or you have no need for it, I'd pull it off just so it's not in the way whenever you need to work on the car and a little neater appearance.

Interior- do whatever makes you happy. If you keep it stock, then you only need to redo the parts that need it.

Why do you want to add headers? Headers on a street car can have a lot of drawbacks. I'd stay with manifolds if it was me. If you really feel the need to do something then go with the long branch manifolds. I don't think you'll see any real gain on a stock motor though.

If the motor runs well, with good compression and doesn't smoke I'd leave it alone. No urgent need to do hardened valve seats on a Pontiac. The heads were made of good iron and hold up pretty well.