I recently replaced my tank because the neck->tank joint was leaking. It only leaked for me when I filled the tank all the way up, or had the rear end of the car downhill.

I bet you had a small leak that just got a lot bigger. The neck is soldered to the tank, and maybe that whole joint let go and the neck isn't pointing inside the tank anymore. Some JB weld putty might get it working enough to get you home. A new tank isn't too bad. I think mine was like $100 or so from Summit. Only issue is that my old cap didn't fit well on the new tank. Otherwise the tank swap was really easy.