TOFIC & MattW , no matter which way you twist , flip or turn this , your enemy is going to be the shape of the classic Bronco. End of story.
It would seem that a good realistic MPG number to shoot for is about 16 mpg.
Physics will prevail in the end. Regarless of how your gear it , power it
* , tranny it , etc, X amount of power is required to move the Bronco at Y speed and it takes Z amount of fuel to generate X power . Z does not really care too much if it goes into 4,6 or 8 cylinders.
Personally , I've been wanting to build a torque monster 351 , built to make its power right at hi-way cruise speeds , using a lot of good parts that will work together to make the power and make it efficiently , EFI , good alum. heads , proper intake , cam and gearing , nv3550 ...etc. But I know full well that it will most likely never get 20 mpg.

The 2001 Superduty 7.3 Powerstroke has been getting eyeballed pretty hard now that the body is getting a fair amount of rot on it though . ;D
* Now diesels on the other hand are somewhat more efficient by their nature , so I'm guessing you could see 25mpg from a variety of diesel motors. The swaps are very involved however. 4BT , 6BT , PSD ... etc.