leak at sending unit

If you don't find any difference in the gaskets, try the method I used. I installed mine with the rubber gasket between tank and sending unit. Then I used some thin gasket material and made another gasket which I installed between the sending unit and the lock ring. It took up the space enough to give a good seal.

Also if you have access to the proper tool to install the lock ring that would be a plus. You can get an even pull on it instead of tapping on one tab.