If I'm understanding the problem correctly the grommet is already in the opening and you're trying to shove the neck through it? If so try it the other way. Put the tank in place and push the grommet over it. With the grommet already in place the rolled lip at the end of the neck may simply be too big to shove through the existing space with the body metal not allowing the grommet to expand enough to allow the passage. I put the grommet on my '38 this way with no issues.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.