I have seen MANY 76-79 F 250s both stick and auto and have NEVER seen one get anywhere near 14 MPG no matter what was done to them or which engine combo was used, mine got 11 with headers, a good intake, carb and cam designed for economy, that was IF I didn't get it over 65 on a flat road, a 12 valve would definitely get you there and likely into the 20s but a lot of work it would be!
