Wow, fascinating stuff! Wonder what became of it?
Reminds me of the '70 GSX prototype that had the custom interior.
Couple of ideas on finding more info:
The Sloan museum in Flint would be where I'd start.
Old car magazines likely covered the big 'new car' shows back then. First step would be to find which cities held them.
There was a Worlds Fair in NY 64/65 with a huge GM exhibit, but it may have been too early to include a '65 GS.