Re: SRT-6 Xfire to 400 HP?
You will probably need, Pulley's, upgraded ECU, Cams, Headers, Exhaust work, and Ported & Polished heads. Removing the headers & heads you can probably get in the 375 rwhp.
There are different routes to get the above with varying prices. Headers will have to be custom as no one makes them for the car.