Not an E body but in my old '76 D100 I got 13mpg highway/10mpg around town.
Stock, lo-po 400 short block, (maybe 8:1? probably more like 7.8:1) early '70s iron heads with small valves, Performer RPM intake, .484" Ricky Racer purple cam, (yep) rebuilt by me Holley 3310, generic headers, dual exhaust into single side pipe, basic electronic ignition, no vacuum advance. NP 435 truck 4 speed trans with granny first (basically 3 usable forward gears) 3.23 gears, 15" wheels. Aero as a brick, no tonneau cover.
I impressed myself with that result. Drove terrible when I first got it - sluggish, massive bog and just hemorrhaging gas. Took a LOT of tinkering to get it to accelerate and cruise smoothly - timing, transfer slots, idle screws, vacuum tuning, power valves, secondary spring... all of it. An A/F ratio gauge was instrumental in the process and was my initiation into EFI.
Was going to change the cam to a milder Hughes "RV" type cam but never got around to it before selling the truck.