Engines of that era were designed with not much thought towards fuel economy as gas was 25 cents a gallon. Here is what I did on my 37 Buick with a straight 8 that clicked out a steady 12.5 to 13 mpg. First a complete tune up with the car on the chassis dyno and O2 sensor in tail pipe. Got it at maximum power and torque which I knew would NOT be maximum economy but gave a point to work from. Then cleaned up the air resistance by removing the sun visor, outside mirrors, trip lights, bumper guards, radio antenna, but did leave the aerodynamic hood ornament. Put on radial tires with psi of 40, replaced all fluids with Torco synthetic, installed a 190 degree T-stat, and replaced mechanical fan with electric. Retained single exhaust with free flowing muffler but your V8 would benefit a little bit with duals. I tried different gear combos and did some "seat of the pants" tuning all of which resulted in a 2 to 2.5 gain in mpg. Gas was near $4 at the time and I was going to be the first one with a 20mpg straight 8---dream on. Car is back to normal now and so am I. Weight, shape, engine air flow efficiency, and piston travel in feet per minute will determine economy and your truck is hurting in three out of the four so don't expect the moon.
