Stoover 007, go big!!!!! I have a '96 F-250 with the 351 and a 5-speed 4x4 3.55 rears. I ordered this truck new thinking I couldn't afford to keep gas in the 460, and the $4500 for the diesel at that time was out of the question. I get 11.5 MPG, uphill, downhill, tailwind, headwind, loaded, empty, pulling, city, and highway. Go for the power, I don't think you will be satisfied with the mileage performance from the 351. Plus you will have a lot more power for everything. Just something to think about, I took a trip from Indiana to Oklahoma City with just the truck, it was roughly 1500 miles round trip. I drove straight through, cruise control set on 75 mph, stopped only for gas, all highway driving. The 351 averaged 12.3 mpg. My father-in-law drove his motor home with a 454 in it and go 13.5, and we went together!! So go figure. The first time my truck went back for some minor warranty the mechanic opened the hood, looked at the motor, laughed, and said "Should have got the 460, more power, same mileage." Who better to know than a mechanic that worked on the them. Good luck.