I bought a 1975 Ford F 150 with a 390 punched 60 over and ended up taking the c6 automatic transmission and I want to keep the body to use as a daily driver in order to maximize fuel economy I want to get these smallest engine possible even if the engine would require slight modifications would not be discouraging IE moving or re welding motor mounts willing to do just about whatever it takes to improve the fuel economy from 8 miles to the gallon where I was at with the engine that was in the body to hopefully in the high twenties

 

Welcome to the site.

Going small/X small on the displacement can have an adverse result, or deliver no realized gains. The body/chassis are fairly heavy and running a small CI engine can just add to more throttle angle needed, or, time under acceleration to get the hulk moving and up to speed. Or, should you decide to tow/haul at some point, have a heavy foot, small doesn't always pay off. A FI 5.0/302 with overdrive, or 5sd manual/OD may be a decent choice and can be had reasonably cheap. A sizable job and some unexpected costs involved for most swaps. We used to retrofit 70's 250/350 with Perkins and Detroit diesels and was a big job as well. Never saw fuel numbers in the top end of 20 either.
'High' 20's,, wouldn't be a realistic number given the weight IMO unless a diesel was sourced or smaller CI/FI with OD, and even then, or without OD, gear set change possibly, doubtful. Lower 20's,, doable with gas,diesel without extensive work, or a fat wallet. There was an old saying about HP and what you can build?,, the answer was always 'how much money do ya have! Same goes for MPG..