No need to reuse any old parts or make another gasket. This is a real common problem. All you have to do is look closely at the retainer ring for the sending unit. The "tangs" that tighten down on it as you turn it to secure it need to be carefully bent just a little bit before you install it. this puts the ring under more tension and causes it to compress the o-ring like it should. I've done it a good bit and it seals right up every time.[/QUOTE
I just re-read the OP and because you have a new tank, Strokers method should do the trick. I bent the tangs on mine and no help. I was using the original tank and just couldn't compensate for it, that's why I had to use another piece of gasket for a spacer if you will. The metal on the sending unit was a tad thinner than the original as well as the lock ring. The metal around the opening was weak and stretched so I had to resort to a hickabilly repair.