Looking through the SAE specs that GM Archives allows you to download, I'm surprised to see that they don't actually list the cap type.
What I would expect is a non-vented cap on any with the low horizontal tank and the vent loop that goes up into the trunk. On wagons with the quarter panel tank & inlet I suspect the cap may be vented. It may depend on year. Certainly by 1971 (1970 in California) the fuel system for all was closed and would not vent to the atmosphere.