If you look between the spark plugs on any fe head you will find the date code like 68ae meaning the heads was produced in 1968 . Also for a 352 you find the numbers you put up . For a 390 the numbers will be 6090-G ..for a 406 6090-C for a 3 duce 406 , 6090-S for a 3 duce t-bird ...6090-N for cj heads and 6090-H for 427 replacment head 6090-F for mid riser , 6090-J high riser . There are more for 427 as well The power transforms a great deal in hp from the first to the last I described .
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