Most prototypes of the muslce car era could not be licensed or insured as they were not DOT legal. They were unceremoniously crushed after the car show season
The prototype Buick GSX was an exception. Fully street legal with a Stage 1 455. This car was a star of the auto show circuit, with a special pearlescent paint job and boldly contrasting accent colors. After the show season was over and it was briefly on display at a dealership, it was sold through the dealer and survived to modern day although it needed restoration; Buick wasn't interested in keeping it and neither was GM. The Holy Grail of Buick muscle: out the door, with non-production parts!
http://www.buickperformanceclub.com/GSXPrototype.htm (the captions do not really even do justice to the non-production parts: hood scoops are completely different, front airdam shaped differently, seat covers were not "custom", they were one-off for the prototype, etc)
This link shows production '70 GSX examples
http://www.buickgsx.net/1970GSXPIC.htm