I was able to push the grommet I linked you above into my new tank with my hands. I think I had to use a screwdriver to get the last little bit in. If you're trying to do this without dropping the tank you will need to remove the filler neck in order to scrape out the old sealant and install the grommet. I recommend dropping the tank to do this.
Lube the inside of the grommet after it's installed and the bottom part of the filler neck with dielectric grease (which is silicone).
