agreed with pete.

Edlebrock "air Gap" or performer or a Offenhauser but make sure you get a "dual plane" manifold for a street use car. I'm still assuming this 400 is a US block & not a Chevy 400??? Dual plane is two levels under the carb base. When you look into manifold where carb sits one side will be a little deeper than the other side, this seems to work better on the street!!! I'd say leave you exhaust the way it is,no sense throwing $$$ away. I've got 2 1/4" on my 64 Catalina, still running stock manifolds [ cause no body makes a header to fit the body style] without pounding the "crap" out of them to get a "fit" a set of "mid" 3's gears out back, mid range CAM & you should have a "decent" performing car. You should find out what size Holley's on it, some guys go with the "bigger is better" but an over carburated engine won't RUN worth a CRAP!!!!!

Carl, "at least ' 20sec. "quicker" in the 1/4 mi" Honestly I don't know but the throttle response will be noticable, & maybe a bit of fuel milage gain. What you want to achieve here is to get the engine to run in it's best "power" zone, get it to "Breathe" The US Pontiac 400 had a very poor Intake on it.