Hindsight is 20/20

Most likely what my5thmopar said (but check the sensor hardware also for a leak as mentioned above). When I installed my new tank, I used the old clip ring with the new gasket and made extra sure that it was installed straight and centered, and I have no leak with the tank sitting full. There is a sending unit lock ring tool (Vans Auto has it, or other places) that isn't necessary, but sure does work great for removal of the old one and installation of the new one. It's like 12 bucks. I sprung for it and didn't regret it because it made short work of it. Good luck!

EDIT: And make sure you get the ground strap back on. If you need a new one, watch out for the ones usually sold, like by Vans... it's too short and won't work. Do a search here and read about different solutions, but it's pretty easy to just make something that will work.