440 hone re ring mild 220--226 duration cam, Qjet carb, 14degrees initial timing 36 total all in by 2800 12 degree vacuum advance for a total of 48 to 50 at cruise and 60 mph will get you 15ish mpg. The holly double pumper on a truck will just use fuel to use fuel. Yes you could tune it to do really well BUT can you tune yourself to be fuel efficient using it?.
2nd this approach. I had a similar build; 440, +.030 with tight quench, smallish 224 cam, blueprinted chambers with mild porting, port matched intake, dry film lube and thermal barrier coatings everywhere, headers, with carb (AFB) and ignition tuning to the Nth degree. This sat in a '73 crew cab, 2wd, 3 spd auto, 4:10 gears, 33" tires that was typically operating at 4500-10000 ft in elevation. The thing was a beast. I could pass traffic, going up passes, with a loaded trailer. It averaged low teens loaded or around town and high teens unloaded on the highway.
But, I'd agree with others that a lift kit and 35s isn't going to be stingy with the gas and isn't the best tow vehicle around.
