Fuel tank filler tube installation trouble

My son has a 71 Swinger as well. He had the same fight with a new tank, grommet was to small. I can't remember
if he modified the grommet or not. I will talk to him today and get more info. I know he used silicone grease.
I talked to my son. He fought it until he got the tube in. He didn't modify the grommet.