How rare is a 68 GSS?

Blownfish said
I know of one locally in St. Louis that got turned into a pro street back in the 80's. A friend of a friend owned it, never understood why he insisted on tubbing such a rare car. I ended up with the original spare tire out of it, never been on the ground. I still have it, was always going to put it in my 68 as a conversation piece at the shows.

That's a neat story to tell but if I were listening to it I would ask how you could prove the spare tire we are looking at in your trunk is out of a GSS compared to any other Chrysler car from '68?