DannyP, do you know exactly what pushrods you were using? I ordered a set of the Aircooled.net heavy duty aluminum pushrods. They are supposed to be good to 500 psi spring pressure.
Also, our cams have the same duration, but you are running a ton more lift. I opted for less lift to reduce the wear on the valve guides. There is very little flow difference on ported heads at .500 lift and .600 lift.