Engine size recommendation

The 318 will have a mileage edge, no doubt. I'd rather go with a 360 for added brute torque down low.

A basic rebuild with OE pistons or upgraded compresion units for more compresion can be used with gapless rings. No blow by, max compresion.

OE heads simply rebuilt. A upgrade would be 1.60 exhaust valves.

600 cfm carb or a TQ, small primary version on top of a Edel. Performer intake.
Headers a plus, dual exhaust off the manifolds for sure. 2-1/4 min, 2-1/2 max.

If your not sticking to the stock cam, I would go no larger than a 216/218 intake duration and 8 - 10* more exhaust duration with stock port heads.

I ran this cam in a Duster with 3.21 gears and 225/70/14 tires. The engine was a stock '79 - 318 with a LD4B intake and 600 carb, headers w/2-1/2 exhaust.

You should be able to get 18 mpg's with added compresion, use of a MSD 6 box for making sure all the fuel is burned and taller tires would help in mileage as well.