I think getting the neck to pop through the grommet was tough. And since it has a metal face plate on the outside that plate would smack back against the side of the car. Probably will just use the epoxy to get me through Carlisle then pull it down and fix it right. I'm a little concerned how hard it will be to break loose all that epoxy junk.
By grommet do you mean the rubber piece in the trunk? That is just a cover. Take the screws out and slide it up the filler neck so it's out of the way. You will then be able to remove and reinstall the filler neck much easier.
as @g413 said don't bother with additional sealer or anything like that, remove all of the old sealer.
Be careful when getting a new grommet, some of them aren't pliable enough almost like a soft plastic, and can be a pain to get the filler neck into. This is the one I found on ebay and it seems to work fine. I test fit my filler neck in it (I haven't installed the new tank and filler neck yet) 1967 - 1976 A Body MOPAR FUEL Grommet Gas Tank Seal CORRECT FIT New | eBay
After installing the grommet into the tank lube it as well as the bottom part of the filler neck with a lube, preferably silicone based
