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I have little direct experience in the issue, but the second hand hearsay I can relate is that apparently the E heads flow a little bit better in stock form, but the late F heads have more metal in the ports and can be worked more.
To tell them apart, look at the spot where the middle head bolt on the manifold side seats. It should have a little flat machined 'island'. If the island is completely surrounded by lower, rough cast metal, then it's an 'E' head. If it is connected to one of the nearby injector pads with a thin raised machined strip of metal, it's an early F head. And if it is connected to the injector pads on BOTH sides with thin raised machined strips, then it's a late F head.
Another consideration in how much a head can be worked is core shift. the internal passages were created via an inserted core in the casting process, due to production variances these cores were sometimes out of place. The head may have benn cast well enough to make a stock head, but if you start grinding metal out of the ports it's possible to hit air prematurely if the core is shifted. I'm not really sure how to tell if a head has (bad) core shift or not.
In stock form, the main difference between the E and F heads is merely the machined height. The F's are taller, thus having deeper combustion chambers with more volume, and thus lower CR. ~35 years on from their manufacture, it's possible any head you get has been shaved down some unknown amount by now. It's probably best to CC the chambers and see what you've got.
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'63 PV544 rat rod, '93 Classic #1141 245 +t
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