SWMBO's coupe has 64 heads. Feb and Mar casting dates. Anyone know when the valve size change was? I have a matching date code 66 that I'm going to rebuild to swap out. Just wondering about the 64's before tear down. BTW: one of the PO's had installed rail rockers on these slot heads. I have always heard that is a no-no. The rockers make an angle from the push rod to the valve stem, is that why?