There is an earlier prototype in The Henry Ford Museum in Dearborn Michigan.
They call it the Mustang I. To me it looks like a much smaller GT40.
1965 Ford GT40 Roadster Prototype
The origin of the Ford GT40 is perhaps one of the greatest stories in all of automobiledom: It was a four-wheeled “screw you” delivered from Henry Ford II to Enzo Ferrari. (Photo courtesy of Pawel Litwinski | RM Auctions)