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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..