Howdy neighbor, I'm in Media PA.
Will the new motor be a rebuild of the original or a newer more efficient injected one? Does the Bronco have an auto or stick and is it an overdrive unit? If not swapping to an OD trans will help to move you to your target.
One of my rides is an '05 Ford Excursion V-10 4X4, it's modded some to improve it's towing abilities as it serves as our dedicated tow rig for an 11,000lb travel trailer. But in it's original factory stock configuration I have gotten 15.5MPG on a mixed highway / coastal Maine town trip with 6 adults onboard, not bad for a giant 8,000lb wagon, your goal should be attainable.
