You are going to get 100 different answers to the fuel milage question. I would bet no more then about 20 at the best and most likely about 15 to 17. The truck is pushing a ton of air so no matter what engine you give it, its going to have to work harder then a modern truck would. My 250 with automatic and 3.08 gear gets 18 at 70 mph in a 3300 lb truck.
Joe
