I really have no idea, I would imagine the 400 can take the most horsepower but like I said, I don't know squat about automatics and don't like them anyway!!!! Tranny shop will warranty the 350 for the torque the engine will be making and that's good enough for me. As for the wieght, not sure but somewhere I've heard the 400 is about 30 pounds heavier. With a nip here and a tuck there I've managed to take about 130 pounds off the car, hate to give it all back on the trans and converter.....