You definitely need to upgrade the valve springs on the P heads. Ford racing, trick flow, comp cams, Anderson ford motorsports all make upgraded springs for stock cast iron ford heads. I've got the trick flow 2500100 springs on mine. And are good up to a .540 lift. Check out latemodel restoration, American muscle, ford racing or any aftermarket parts suppliers and they'll have what ya need. And always check push rod length, and push rod to valve clearance whenever doing any upgrade.