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...