Fill neck grommet issue

I got fuel neck grommets for my '69 Dart from Classic Industries and from Mancini's and they are both too thick to get the tube
into the tank. I can get the old one in but the aftermarket ones (I'm guessing) just seem to be hard and not flexible enough to go in.
I'm asking if anyone has had the same problem and if you didn't, where did you buy it? Thanks..... John.