Go with the 1407 750, your 383 will thank you. Put one on my 340 Demon and it loves it. The main reason people get worse mileage with the 750 is because they love the power increase and are standing on the throttle more. The 383 Magnum wasn't intended to be a gas miser so let it breathe like it wants to. If you want better gas mileage put a 2bbl. on it.