I assume that it was not recommended to go above .450 lift because that motor has Vortec heads on it. If this is the case, you will have to get bigger springs to accomodate a more aggressive cam. In order to put a bigger spring in the head, you will have to get the spring seat machined out to fit a bigger spring. Once you get that done, I would use the Comp XE274H cam. I had the XE268H cam in a 355 with Vortec heads that had the spring upgrade, and although the motor ran very well, it would have benefitted from a little bigger cam.