I am semi restoring a 1966 Dodge Coronet Deluxe to make it a good driver, I bought some new replacement parts from Auto body Specialties back in 2014 including a new gas tank and 3/8 sender. I put 10 gallons of pump fuel into it the first time last Friday and found out over the weekend that it leaks around the new fuel filler grommet really bad I haven't contact them due to the time differences from them to out here.
My question is this common or do I have a bad tank?
Thanks in advance for any comments or suggestions

New grommet?? If it is new and fully seated onto the lip on the fuel tank, unless the filler tube isn't lined up properly, there should be no reason to leak. A little grease on the grommet sometimes helps to let things go into place better. I just did the tank on my 66 Satellite....used grease on it and made sure the tube was spotless where it goes into the grommet. There are a couple of different grommets too...You did get the right one...didn't you??

Yes, the gasket makes all the difference. On my 66 Coronet I use the one on the right in the picture below. Never a leak but it was on the original tank. Part number is 3404182. Seen repro ones go for $14. Original ones still show at the dealers for $30+.

Its been a while but if I remember correctly....

Based on the above pic the grommet on the left looks like the original that came out of my car that was NOT being reproduced when I ordered the part for my car... I used the part on the left ( I believe it was for 68/up) and it worked great with my new tank ( never had a problem).
I did lube up the fill tube when I installed it in the grommet/fuel tank....

Those 2 grommets are exactly the same except for the extra "splash lip" that was added (the one on the right) in 1970. The one on the right will fit any 66-74 B Body but was "correct" starting in 70.

The ones the dealers sell aren't NOS. Probably a good thing. $30 is EXPENSIVE but at least you'll be sure they're made from the right materials. Tank grommets made from the wrong material was a problem in the past and has reared its ugly head again. I'd check to see if a leaking tank was from a grommet that "swelled". OUR tank grommets are made in OHIO (Akron, Ohio is the rubber capital of the world) and made from the correct material; Nitrile.

Quick note; I've seen some grommets being sold that looked "burned". Not a good sign. Also they should have NO SMELL. None. Not a cheap rubber smell. Not a plastic smell. Not a burned smell. If in doubt throw in a can of gasoline. They will swell noticeably in a day or two if the wrong material