Pat68HO,
The classes are dynamic. The GTOAA will make them up based on registrations (they could combine some years or break a certain year into multiple classes for example). The only decision you need to make when you register is if your car is "stock" or "modified". I must refer you to the GTOAA for class information. See
http://www.gtoaa.org/pdf/popvotechart.pdf
When you arrive, you will go through a tech inspection to determine if your car is stock or modified. Please take the time to determine if you think it is one or the other before your register. It will save a lot of time in tech if the class is correct (68 stock or 68 modified).