The Mustang Boss 429 was available for two years only; 1969 and 1970. It is very rare, with only 857 built in 1969, and 499 in 1970. They were built to qualify the engine for racing. It was rated at 375 horsepower, and everyone knew that this was a gross underrating. I've been lucky to see two since 1969, and only one of those was on the street.