'63 valiant 4 door /6 cruiser questions.

Greg
I start with the last first. Dont forget the windshield wiper pivot seals( Schumaker makes these) I have a Duster you would think I left rolled down but the wiper seals are all it is. Also check the cowl vents (fresh air Vents). You car you can take the plugs out of the fire wall and get a wire down and/or a small vac hose.
As far as the exhaust just Y pipe and run 2.25 inch all the way to your muffler then 2" out is fine. Anything larger and you will nix the benifits.
Good way to work on it is a piece at the time. I SOOOO wish I would do that instead of seeming to get stuck in these endless projects.
Frank