Mileage depends on more than just the motor, itself. Total vehicle weight, tranny and rear gearing, aerodynamic drag, emissions controls, etc. ...
An H.O. EFI 302 dropped into a Box-bodied Panther car ('80's Crown Vic, GrandMarquis, etc.) might get 21 MPG, tops. The same motor in my notch gets 23 to 24 MPG - lighter car, T-5 tranny, not shaped like a brick, etc. So, it's feasable ...
...but I don't see it happening in a Bronco versus an F-Series truck.
I'd go with the Bronco, myself, because it's more versatile. You can tow with it and so forth, it's more liveable for day-to-day driving, and it's a little better suited for off-road uses than the ol' pick-em-up truck. Plus, it's newer.
