Replace the 318 with a Slant

If your getting high 20's outta the /6 in the Duster/Valaint and need the mileage, leave it in there. IDK about swaping it in to a V8 K frame though. Maybe one of the /6 nutz know how to do that.

The truck is higher up in the air and has more wind resistance. Bad for mileage. Leave the 318 in there for the ability to get the truck moving well and quicker since the 318 has more torque.

For a small price, the basic tips for mileage can be used on the 318 for slightly better mileage.
I'd start with a;

MSD ignition, or equal.

Headers for trucks are easy to get and can be had inexpensivly.

Dual exhaust you can do yourself easy. A simple system of striaght pipes from the header into the mufflers and a 90* bend to exit out the side by the rear wheels can be done very cheaply. Inexpensive mufflers like a set of Thrush's will be quite enuff for most everybodies neighbor.

Super tune the carb you have now.