Fuel Tank Problems

[Tommy S.]

I recently had to replace the gas tank in a 1986 Pontiac Parisienne. The first time I filled it up everything worked fine. The second time, the gas pump failed to kick off when the tank was full and gas leaked out around the sending unit O-ring. Gas never did overflow the filler neck. There appears to be no way to tighten the sending unit beyond turning the locking ring that secures it. How leak proof are these supposed to be and what can I do about it? Thanks for any advice that you can offer.
Kindest Regards,
Tommie in Atlanta";