Id drop the tank, its not hard. replace the sending unit and the o-ring make sure the locking ring stays centered.

But before you put the tank back in, fill it with water and a little detergent. rinse it out a few times and resolder the neck back on with a propane torch. its real easy. i used to do these 1 every couple of months when i worked at the Texaco station.

BTW , trivia.. the Texaco station i worked at is in southhampton MA. it was the station the movie who's afraid of virginia wolf the garage scenes were shot there and at the right next door at Red Basket - 151 College Highway with richard burton and elizabeth taylor. They had signed pix all over. the movie was shot way before i worked there 1966, i was 6.
it was a cool garage , the owners dad whom had passed away left it to him. The only reason his dad built the garage was to have a place to restore his old cars . They had stacks of model t and b engines,frames body parts ,etc there was a terraplane, hupmoble, and a 1912 cadillac in almost pefect condition (stored inside) somebody from california bought it for 100k back in 1979. there was NOS parts for a ton of old cars. they sold it all off and made a bundle.