Believe your asking so you can choose between the large rivet versions that are being reproduced
If you want to be original then neither. Appears that NJ that year used what is referred to as the "Dearborn" radiator cap with the small rivet.
Here are three examples of the cap from 68 NJ cars.
Suggest you purchase one of the incorrect ones with the brass rivet as a place holder until you can find a correct one.