I can tell you after driving my '37 for 18 years now, wind and speed hurt the mileage more then you think. Drive at 55 and it will go up, hit a head wind, it drops a lot. The design of the truck looks good, but aerodynamically it's terrible. The straight up windshield and big fenders really hurt it.
Now for the engine, if the overdrive brings the rpm down under 2400 or so at highway speeds, you mileage will suffer. Even my " late " model 250 likes 2500 -2700 rpm out on the road. I would guess you are running well below the optimal torque of the engine. Run around in 4 gear for a while and see what the mileage does. I run a 3.08 gear with 28" tall tires and automatic trans, 1 to 1 final drive, at 65 mph the engine runs 2600 rpm, this will get me about 21 mpg if the wind is not to strong. As Jeff mentioned, if you fuel has more then 10 to 15% ethanol your mileage will suffer.
