I think I can now answer my own question. This is definitely Courtland, Alabama May, 1966. I am certain of this because of the other cars present in other photos that are on the entry list for that weekend. An example being the George Barber Brabham BT-8 which is visible behind the other cars, next to the Barber van.
Looking at my own picture in more detail, and my higher resolution version, I can read Dockery Ford, Morristown NJ on the sail panel. So with your input I believe this to be 5R100.
I do not see this particular car in online action pictures so it may not have run at all during the weekend or, more likely, ran a Saturday race but not the Sunday race.