I'm looking at a 1979 F150 4x4 for my 16 year old. It only has 47,000 original miles on it, but it has a 400 with a 4 spd and I am concerned about the mileage it will get. Would swapping the 400 for a 351 or 302 improve the mileage noticably? Or is there maybe another route to go to improve the mileage, i.e. changing trans? When he's older and can afford the gas, we could always go back to the 400. Thanks for any suggestions.