SuperBlue, I put the grommet in the tank first, then tried (or more appropriately I'm still trying) to put the filler neck in. I don't think that I could ever get the grommet in with the neck in place. It was hard enough to get it in without the extra girth that the neck adds. Ultimately, I have to just fight this thing in place I suppose. I already tried to slick up the neck with steel wool and a little windex which didn't work though it did pretty the tube up a bit. My next try will be with a little silicone grease that is used for maintaining seals on my scuba equipment. Hopefully, that will add enough slick to let me force the neck through. I gotta say though that this is a really big pain in the butt for something that looks like it should really be easy.
I had the same issue. Before install I did a test fit out of the car, and it went in no problem. Put the tank in the car, and the filler neck wouldn't go in...
Took it back out, it fit fine....
Put it back in, and the filler neck would not go in with all my might....
Long story short, the angle is very poor when you are first starting the install, so if you loosen the gas tank hangers, and let the gas tank hang down a bit so that you get a better angle, and the filler neck should slide right in.
To sum up my day in smilies...