Welcome to CHR, good having you with us! About all I've ever gotten out of an FE for mileage was in the 12-14 mpg range. Mileage just was not a consideration when these engines were designed!!! Back when the F-250's with an FE engines were the truck to be towing with we had a few different combinations but that was about the best we got----and if you left late for the track and had to hurry, that number would drop to the 8-10 mpg range! Another consideration would be to build the engine for torque and mileage, then put in a Gearvendor's Overdrive unit to scrub off a lot of RPM!

If the truck's a keeper and towing power along with good mileage is your criteria, I'd suggest save up some coin and find a take-out 7.3L Ford Diesel with a 5 speed behind it! My current toter has that combination and exceeds your 13-14 mpg by a considerable amount, with the trailer hooked up and the AC on! I'm sure such a take-out diesel in your "neighborhood" would be quite expensive, but still probably worth the investment if the truck is a keeper!!!!!