Actually St. Louis only applied undercoater very sparingly to minimal areas of the inner wheel side of the rear wheel wells back in '68. This was only to cut down on road noise.
Although most Vettes had some applied, it was a very popular dealer applied extra cost markup item.
With that said, The Eastwood #16007Z http://www.eastwood.com/ew-rubberize...sol-18-oz.html
Will work fine and look great.