Hardtops use the same quarter glass....coupes use different frames and glass.Even the inner steel upper panel that screws on is different between Dodge and Plymouths.The main difference between the two is the way the quarter panel is shaped at the belt line which is why the option belt mouldings and drip trough end cap moulding are different between R's and W's.Pretty minor ,but it's there.Chrysler must have had lots of money to burn through back then.