1978--Fuel gauge is 'stuck' on "full", regardless of amount of gas. Sending unit problem?

my 2000 buick regal ls. fuel gauge says full until the tank gets below half then comes down a little from the full mark. they said it was the sending unit but actually I changed it twice bought the fuel pump and sending unit and this did not work returned it and got another still didn`t work right. I was welding on the exhaust and forgot to remove the battery cable, maybe I magnetised the fuel gauge. but to this date after months it still dont work right anymore

Chances are the float inside the tank is stuck in the up position and doesn't come down as fuel is used. But before removing the tank to check this, see if something simple like a bad ground to the sending unit at the tank could be the problem. Rust on these old cars can cause a lot of bad stuff. Also have a competent mechanic check the fuel gauge itself before dropping that heavy gas tank! In the meantime, just top off the tank every 150 to 200 miles.