Ive had my. gas tank up and down a dozen times in my quest to get the sending unit to read correctly and so what I know is the filler neck grommet you have tube careful that it doesn't get damaged or doesn't seat properly
As.far as the sending unit a good rubber gasket is needed but not all locking rings are the same thickness.....Better quality locking rings are slightly thicker steel and this puts more pressure on the rubber seal
Im sorry cant remember were got my locking ring, but I know the cheap China ones are thinner material . That grounding strap also has to be tight and clean, a good ground is the first. thing to check