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+.
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
