both engines moving the same weight the smaller engine is gonna lose out in the MPG department (if your talking american made, same does not apply to toyota).
That is....if you keep the foot outta the throttle.
i had a 302 in a f100 and 351 in numerous bronco's anywhere between 8-15, never better than 15.
i would say go with a 351 or an inline 6 if you can find one.