I can't recall exactly, but wouldn't you be able to make a measurement with a big pair of calipers between the machined sides of the heads?
You could compare it to one of the sets of 4V heads I am sure you have laying around in your garage.
It wouldn't tell you FOR SURE that any additional thickness you might measure is on the intake valley side of the head, but it might give you a starting point to continue your investigations....
(Not that it will solve the problem, either!)
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