You can get 450 Rear Wheel Horsepower, and a whole lot more, easily with a supercharger.
If you want to go above the 450 RWHP it would be smart to get forged internals or you will risk breaking the crank, rods or pistons.
supercharger will get you to 400 at least the other 50 horses will need to come from a cold air intake system and a new set of performance spark plugs. you could also install an exhaust system that will give you 6 or 7 more horses. good luck!