How do you get 400 hp out of a 4.6 motor?

best of luck to you with that. I would recommend a large turbo and a tuner chip. I would definitely replace the heads with gt heads or something more. It is a small block engine and probably wont handle the torque with out leaving pieces on the road behind you. Also consider the wear on the trans. This engine just isn't designed with that kind of hp and torque.