Can guide plates be used on early 66 289 heads with the slotted holes. I have 2 slotted holes in the heads that the pushrods have widened causing the rockers to uncenter themselves on the valve tips. These heads were redone a few months ago with all new guides and screw in studs. Its possible these 2 slotted holes were already damaged before the heads were done. Will the slotted holes have to be drilled out round in order to use guide plates?