Ramcharger exhaust 2-1/4 duals or single 3"

You should be getting at least 18 MPG out of that. Your Quadrajet with lowered float height lowered to smog 318 settings will help a lot. If you were able to nearly pass emissions without a pair of cat's, a single high flow cat should get you into compliance easily. It takes a 3.25 inch pipe to equal the end surface area of two 2-1/4 inch pipes, so go 3-1/2 with something like this "neighborhood friendly"
Jones Exhaust F626-7D Jones Exhaust Full Boar Full Flow Truck Series Mufflers | Summit Racing
Or if you can stand the drone
Cherry Bomb 87527CB Cherry Bomb Glasspack Mufflers | Summit Racing