I have a bunch of gaskets left over from my days in 64-67 with a GTO 4bbl. I'm like most of us and never throw away anything the at looks remotely useful.
3 of the gaskets I have are like the ones to the top (or is that right) in your picture, They are probably left over from rebuild kits and were intended for some other applications I didn't have. I would have used gaskets from kit like my original if at all possible.
Other gasket I have is like the bottom (or left) in your picture with the two little holes near each secondary hole. I don't remember, nor does the parts book seem to mention a metal plate gasket. I have seen them in other applications but don't remember any for Pontiac in this era. But notice that the slots in the metal plate join the two holes on each side into essentially a single hole. I don't know without more information than I have what the holes are for. But as a guess I would think they are for a manifold vacuum signal to somewhere. Perhaps to power valves but I'm not sure. Are there holes on the bottom of the carb that align with these holes? If so then the other gasket with no holes that align with these would tend to render the holes useless. In this case I would suggest not using that gasket not the metal plate.
As for your gasket with only a single large opening I think that is intended to be used on a single plane manifold. Pontiac factory manifolds tend to be dual plane so should probably use the four hole gaskets.
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