You said long bed 6.5 to 7, To me a long bed is 8 ft, . I have a V10 , 2001 with 362k miles. My brother has had couple of the ford diesels and he has spent a lot more money 1st buying them, 2nd keeping them running, 3rd diesel cost more then gas.
I don't buy a truck for gas mileage. Reliability. Also to me if the bed is shorter than the cab it's just not a truck, real trucks have 8ft beds. haha.

 

The short beds have a shorter gas tank too, 10 gallons less than the long bed.