Three main factors working against more miles per gallon mileage: Engine/drivetrain efficiency, weight, aerodynamics
FE engines are generally inefficient but ... run a 2bbl carb (or EFI like Atomic, E-Street, Powerjection III, or Holley's new Terminator), electronic ignition with a hot spark for a complete burn, headers and a free flowing exhaust system, and port work. Tuning to maximize efficiency and A/F mixture but an EFI system with an O2 sensor can do it for you. Reduce parasitic drivetrain drag caused by wide tires, sticky tires, low tire air pressures, and heavy-bodied lubricants. Change the rear gearing to something more friendly like a 3.00 or 2.79 ratio.
Weight... these things have alot of heavy steel.. it's mostly steel! A dock bumper weighs 75 lbs.. a cast iron intake weight about 70 lbs. Lose or reduce as much as you can. Remove or leave empty auxiliary gas tanks. Compare your rig to modern trucks' plastic interiors and tailgates, aluminum tranny cases and wheels, aluminum suspension components, foam-filled bumpers with plastic fascias, etc.
Aerodynamics... face it. These rigs are like sheets of plywood against oncoming air. ...Seal up body gaps, shave any appendages, install more aero-friendly components like 'sport' mirrors, block off the unused portion of the grill/grill inserts, add an air dam (ever really look under yer truck in relation to on-coming air? There's tons of drag and turbulence-creating parts!), bleed off engine compartment air with a reverse hood scoop or hood vents, , add an undertray, and lower the ride height. In other words, "Slick'er up".
Driving technique and speed is also important. Avoid jackrabbit starts. Anticipate stop signs and traffic signals. Be off-throttle as much as you can. Google search "hypermiling" for additional techniques. Drive no more than 60 mph. It takes significantly more energy (thus HP) as speed is increased... in fact, it's an exponential amount - not linear due to overcoming drag. I suggest researching what HP levels necessary to go 100, 200, and 300 mph at Bonneville.. it's enlightening.
My brother can get 15 hwy mpg with his 360/C6/3.25-geared 1969 F-100. I'm aiming for 20 mpg with my 70 F100... it's an experiment - a very expensive experiment but it's my goal. I've got an E-Street kit already and a TKO-500 should be arriving in the fall.
Lastly, 10 mpg at $4 per gallon is 40 cents per mile. Therefore, do the easiest and cheapest mods for more bang for the buck.
70 F100 LB 2WD, 360FE, E-Street EFI, TKO-500, 70K original miles.. follow my rebuild: The Lo-Buck Bumpside
71 F250 LB, 2WD, 360FE, T18, PS, PB, D60 with 4.11s
73 F100 SB 4WD, 390FE, NP435, +4 on 35s
01 SuperCrew 4WD
01 PT Cruiser (DD)
71 Pontiac Lemans Sport Convertible, 350P, TH350, Full Hotchkis/Bilstein Suspension, 17" VN-500s
68 Mustang coupe, 200-1V, C4, Granada discs, 8-inch rear, AR Torque Thrusts
65 Mustang coupe, 289-4V, T-10, SSBC discs, 8-inch rear, Magnum 500s
...looking for a 96/97 Ferrari F355 Spider