Fuel guage reads FULL !!

I just replaced my fuel tank and sending unit, As the original tank was rusted and the guage didn't work.. Now the guage reads PAST full . I know that there is only a half tank or so. Also when I reconnected the wire from the trunk to the sending unit, is was sort of loose, so I tightened the stud some, That was a big mistake. I cranked it down to tight, and the small rubber around the threaded stud is leaking very slowly. Not too good ! What is causing the gauge to read so high ? and do I have to replace the whole sending unit, or can I just replace the small (dime sized) rubber where the wire connects to the sender.