I think this is probably the easy answer:
The Z Store! Nissan-Datsun 240Z-260Z-280Z-280ZX-300ZX(Z31/Z32)-350Z-370Z Parts: Authentic Vertical Doors, 70-75 240Z-260Z-280Z
have you got the standard door seals fitted or aftermarket ones which can make it hard to shut the door without having to slam it hard