Yes I did call them directly and they said they are designing one to fit the 68 as we speak and it will be ready for the market by December.Since you are asking about an aftermarket AC system asking Vintage Air why they feel their system wont fit a 1968 is the best way to get the answer. I'm sure if it fit without modification they would be more than happy to market it that way.
On second thought I'm not sure what you are looking at. I just downloaded their 2019 catalog and the Pontiac GTO page show a 1968 kit and a 1969 to 1972 kit. The page does say Coming Soon for all of them. Did you call them and they didnt know when the 1968 would be available?