What Color for 68 GT?

Being an old Vette guy, I would suggest 69 Vette Tuxedo Black. If you want red, I would buy 63 Vette Riverside Red. VERY, very nice shade of red.
Use a urethane base/clear system. Local paint jobber can mix the color, and put together a whole kit for ya from primers to clear.
Good Luck with the Dart.

George