you'll lose higher rpm flow. It will run good, but won't perform optimal. Stock 318 heads have no problem handling a .455 lift cam. In fact, I've gone .474 with stock 318 heads. You lose lift in factory geometry. IF you go over .455 lift, be sure and check the clearance of coil bind and that your keepers/retainers don't bottom out on the guide- with room for the valve seal