318 vs. 360?

A small block can get the mileage and if it's in a light enuff car, run the time with nothing more than a small cam. Like for example a comp 260. (Realize I'm at sea level.)
I had a 360 get 20 mpgs without headers in a '79 Magnum. But it also had 2.76 gears. (26 inch tires)
An overdrive would be great if possible to obtain cheaply. You'll need to shorten the driveshaft.

I like the 360 better due to the increased crank throw. More torque. For pure street or street strip, there fantastic.
Pistons are cheaper than a 318. Thats about the only real cost savings over the 318.

The 400 (A '78 vintage "P" code engine.) in my current Magnum gets the best of about 13 on the Hwy. Rear gears are 3.21 w/26 inch tires. All stock except headers w/2-1/2 exhaust. The big block is naturaly thirsty.
With the 400, you could copy the Road Runner/Magnum engine to start and upgrade the short comings of the old stock parts for power and mileage helpers.

Things like a;

A T-Q over the Holley they came with.
Open air cleaner
Headers
Mild port work with better S/S valves
Jacobs ignition (Mileage Master)
Electric fan(s)
etc...