Vapor Lock question

Looks like a Winters which is what a Cheetah really is, down underneath

You very well could have a different year tank in the car, that is, anything is possible.

Only thing I know at this point is to get under there and take some photos of the tank area both underneath and in the trunk to see if we can see what's still there.

If this were mine, I'd drop the tank, plug anything left over from the original vent, and braze a fitting into the filler tube and convert it to the 69 / older style.

I'm not exactly sure when Ma changed all this stuff

In 72, you should have a "big can" (vapor separator) sticking up in the trunk with several hoses / tubes going to it. Look at the diagram I posted. In 73, Ma seems to have gone to the later 73/ 74 type which had "all that" in the tank

In opposition to others on here I would NOT vent the cap. This is a great way to get gas splashed all over the side of the car.