456 gears

Dyno headers might make it rich in the car. They are designed to maximize hookup and power. Not fit chassis. So the engine is jetted up for max power, then is all wrong in your car at your track. Also, 7.6 slip is fairly high IMO for a car such as yours, but I still think it's making closer to 500 horsepower than 6. The power you are demonstrating is not what the dyno said.