Not a A-body but a couple of questions about a 318.

1st, put dual exhaust on it, that will give you the most improvement off the bat.

Next go with the LD4B it will run real great on that engine with the stock 318 cam or 340 cam. We put a LD45B on my son's 71 valiant 318 when he first got it. It came with dual exhaust and an edelbrock performer intake. We swapped the LD4B on it and a new edelbrock 625 it ran much better. Stronger off the line with no bog.

If you decide to run headers, make sure that the ones that you choose don't hang too low. They will just end up getting banged up on parking blocks, speed bumps, and curbs. then they will be more restrictive than the stock exhaust manifolds.

I would also recommend using an electric choke if you plan on driving it year round. When adjusted properly, you can drive away in the middle of winter without any hesitation when cold...