New Fuel Tank / Seal yet massive Leak after filling my Tank

So first you need to make absolutely sure it's coming from the filler neck grommet and not a small hole/crack somewhere in the filler neck.

Assuming it is the seal this unfortunately is a common story. There's an abundance of bad filler neck grommets out there, some are hard like plastic and impossible to assemble also. It's a joke.

I checked my ebay history and the one I purchased last year, which has worked perfect thus far, is no longer sold by the seller. It does look like this one however, and this seller, Inline Tube, is reputable:

MOPAR 64-74 A B E C BODY Fuel Gas Tank Filler Neck Tube Rubber Grommet Seal 1pc | eBay

Just note that before installing the filler neck you should make sure the end is smooth so it doesn't tear up the grommet during installation. Use sandpaper or scotch-brite to make it smooth on the end. Then coat it in some grease or silicone before installation.