There are a few things you can do, but fuel mileage on the old trucks was not that great (unless it is a two wheel drive.)
Headers and dual exhaust. Good ignition system (plugs, wires, coil, re-curved dist with vacuum advance adjusted.)
You will need the above with either engine.
Fix any vacuum leaks, tune carb.
Some non-engine related item may help get one or two additional MPG: Redline or other light weight synthetic gear oil in diff. If it is a 4x4 front diff too, depending on transfer case synthetic gear or trans fluid. lockout hubs, properly adjusted and greased wheel bearings, maximum rated tire inflation, correctly adjusted brakes, just the normal stuff.

If you get 15-mpg that would be good.