i am not surprised since you have 4.10 and a camper special. I have a 1979f150 4x4, 390,4bbl, c6, 3.50s, duals exhaust and i get the same mileage as you. I ran a long trip this winter and with a couple wot races in there i got 8.12 on the highway only. The truck may be capable of 9 mpg in the fall with a tailwind. I do think the 4bbl could help you as the primarys are smaller. Hell i bet electronic ignition will benefit you. Just steal a duraspark setup and install.