72 should have had a charcoal canister. I think. It shoulda had factory venting somewhere. Slide under the tank and look up to where the supply hose starts at the sender.There should be an additional steel tube coming from above the tank somewhere close by.It might be off to the passenger side, I can't recall.The early models had a 4-corner venting and later had just a single.
But to answer your question, yes you can buy a vented gas cap.
Be advised tho that the vent has to work in both directions. It needs to allow air into the tank to replace the fuel that is being drawn out during useage,AND it has to vent pressure build-up on hot days.