61-68 had at least 2 options. 16 gal or 18 gal. MOST were 18 gal.
67 the bolt-in configuration changed from 2 bolts on the top to 4.
These tanks have the sending unit in the center of the tank.

69-71 is a completely different version, but possibly 69 retained the older style. These tanks have the sender near the passenger side of tank. They came in several sizes, the most common being 22 or 23 gal. I say 23 cuz that is what I put in my 70 W200 once when I sat in a gas station and ran it out so I could see what size it was. Not sure what the factory spec size is, but there was also a special order 25 gal tank. Ive never seen one of those.