There was only one Mustang SS... the Super Snake, and I just learned the engine in it was pulled out of the GT40 racer that won four successive 24 Hours of Le Mans

The rarest engine to ever feature in an American muscle car is the mighty 427 ci Super Snake V8 featured in the one-off Mustang SS.

Built for a promotional event with tire manufacturer Goodyear, the decision was made to build a special 427 ci V8 for a standard Shelby 500. 

Lifted from the Ford GT40 racer that won four successive 24 Hours of Le Mans events, it produced 520 hp. It would never be used again, making it the rarest engine to ever feature in a production muscle car.

https://www.hotcars.com/rarest-engines-american-muscle-car/