All you can do is make sure the carb is working properly and the truck motor is in tune. In '77 that might mean points, rotor, condensor, and properly setting the ignition timing. I'm not sure if electronic ignition parts were used back in '77? If not, then this truck needs a "tune up" about every 15K miles.
Since it is an old ranch truck, change the plugs, plug wires, distributor cap, rotor, points (if it has them) and reset the timing to specs. If your mpg doesn't improve there isn't much you can do. To change the truck differentials isn't going to yield enough mpg advantage to pay off for the money and time it will take. The stick shift helps in getting more mpg.
Just drive it slow and keep a light foot on the gas petal and you might get it up to 10 to 12 mpg if it is running in good tune.