Either or is going to require some cash, but for drag racing, the automatic is the way to go. If its a drag only,scrap the aod and get a built C4, if you still drive it on the street, have the aod beefed by one of the reputable builders. Contact a local mustang shop for recommendations. As far as the converter goes, call the manufacturer, they can steer you in the right direction depending on your mods and how you intend to drive the car.