The best way to make more hp on a daily driver is with more cubic inches. A very mild 454 will make 400-450 hp. A very slightly rough idling 383 with the right heads and intake will make 400 hp. A rough running moderatly rough running 350 will make 400 hp, but will require lower gears and a slippery torque converter. You could probably make 400 hp on the 383 with a stock converter which will help your milage and make the car more pleasant to drive in stop and go. Use GM's Vortec heads or something similar from the aftermarket.