I wouldn't do it. I don't think you will gain much if any mileage. That truck is a brick in the wind, I think you should be very happy with 15 town / 18 hwy in a 1 ton vehicle.
Your T5 is the weak link right now, so a rear end built to handle a small truck with 120 hp might hold up. What gears are in that mazda rear? Usually the japanese truck rears are 4.10s behind a four cylinder.
What size tires are you running?
