Gas cap dilemma

Could someone clarify for me the proper thing to do here I just want to replace my gas cap with a new one that won’t cause me any problems with fuel delivery. The car is a 1971 Dart Swinger originally a 6 cylinder car now has a 360. As you may have guessed the emission stuff is gone and all the nipples coming out of the original fuel tank are plugged off. This was all done by the previous owner who told me when I bought the car that the gas cap on the car was not correct he just did not get around to ordering the correct one. I thought the MF368 A-Body vented cap from classic Industries would be the ticket since all the emission venting is gone, but as it turned out that cap wasn’t even close. Is the filler neck different between cars? Do I need a special cap for a car that once had an emission canister? I figured the replacement cap would need some venting for everything to work correctly.