FWIW....my '69 half-cab w/ stock '74 250-I6, stock 3-speed, factory 4.10 gears on 35" Dunlop All-Terrrains got 15 mpg it's entire life. Didn't matter much whether it was on the highway or running around the neighborhood...
I'm sure there are lots of folks with numbers for their 302 trucks that can compare to that..unfortunately that 6-banger was the only one of my various EB's that was ever my daily driver where I even cared to figure out what the mileage was.
I 'think' my 408 EB gets about 7 or 8 mpg....rough ballpark, since I never corrected the speedo fpr the gears and tires ;D