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All 289 heads were not the same. There were a number of changes from '64 to '68. To find if your heads are the original type (e.g. '67 heads around the same production date), look in the valley for the valve train. You'll see a three digit casting number. It'll be similar to 7F26 ( like in the pic I've attached). That is the casting date. In this example it translates to June 26, 1967. Your casting dates should not be after your engine assembly date (which is stamped into the block oil pan rail. Generally they're within a month or so of that date.