I bought a new 86, 318, D150 and it got 17-18 MPG dead stock with 3.21's so that's what you can shoot for with it totally stock.
BTW, I have never swapped a 2 bbl to a 4 bbl and gained mileage ever. I've tried it on 318's, 360's, 383's, 400's and they always got worse, TQ, AFB, AVS, Holley it didn't matter and I am a much above average carb. tuner.
Stock 2 bbl, dual exhaust (headers too) and an optimized advance curve along with tight ring seal and good timing set are key elements to fuel economy mechanical wise IMO.

Sheldon