The Dan Gurney Special decal may well be original to the car. This was a dress up option that was available for the 1968 Cougars. It consisted mostly of a different hubcap, some chrome dress up items on the engine and the window decal. While it is an interesting option it really adds very little value to the car. To know for sure whether or not your car left the factory with this option you would need to either have the original dealer paperwork or order a Marti report.
Randy Goodling
CCOA #95