Any hot tips on replacing gas tank filler tube grommet?

I have a 63 dart wagon and I started replacing the filler tube grommet and 1) I have the wrong grommet its too big. not by much 2) I'm trying to put it back together while I find the right grommet and other than patience and perseverance what is the trick to fitting this gas tank back up in there and still getting the tube down into the tank.