If your trying to pull mileage out of a 318 2bbl upgrade to electronic ignition or better still a computer ignition system like MSD or Jacobs (Jacobs mileage master comes to mind)... rebuilt your front end, get steel belted radial tires and run them at max pressure...
I have a 318 B-Body and I have gotten before (and still get) 15-16 city and 20-22 on the highway.... (hiway mileage is ONLY if I do the speed limit)above that (say doing 70-75) and I won't break 20mpg at all!!!!)
In either case b-body or A-body your not going to do much better than that...
Remember (points aside)an A-Body 318 or B-Body 318 wieghts in the range of 3300+ pounds...