Looking good man!
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Yes, you are correct but all the pre '66' heads had the slot. Very common back then when running a big cam to swap in the threaded studs. You can't even tell by the casting number. Got a set of C5AE heads, same casting as the Hi-Po's, in the basement right now. Hi-Po's have cast in spring pockets and what they call a "double dot 289" cast in between the rockers. Another number only the Hi-Po's have is a number 19,20 or 21 cast into the outside corner where the valve cover sits.check look at the pushrod holes if they are hi po the holes will be slotted and the rocker studs will be screw in not pressed in.
if they aren't hi po no big deal they will still work but if they are hi po don't hurt them cause they are worth big$$$$.