Take a look at this topic;
"The Nissan gaskets #4 and #30 are NLA and you would not want to use them anyway as they were open-cell foam that held water and contributed to the rust problems. I used 5/16 thick X 3/4 wide closed-cell foam weatherstrip for these two spots - a perfect seal when the fender was bolted up."
There is also a rubber seal that runs along the top and is held in place with clips;
These are available from suppliers such as the Z Store;
Inner Fender Weatherstrip, 70-78 240Z-260Z-280Z US $29.95 each (2 Required)
Inner Fender Weatherstrip Clips, 70-78 240Z-260Z-280Z US$6.10 each (12 required)