Horsepower with mileage options

Our truck gets better mpg in town and on the highway than does the valiant. I think an od transmission is a great idea, I know we picked up a 3.91 headset for it initialy, but when he decided milege was more important to what he was looking for he started thinking the 3.55 rearend would be agreeable with the end goal. Obviously not as good as the 2.76 gets for mileage, but I think with a 4th gear it would be a good compromise.