The AC Cobra Mark III started life as a British roadster body flown to Los Angeles with an empty engine bay, and ended up as the car that forced a national government to invent a speed limit. Along the way it collected a Jaguar differential, an MGB steering rack, a Volkswagen steering column, and a Ford 427 side-oiler that made 425 horsepower. One example later sold for $5.5 million. Read on for how a hurried eleventh-hour decision produced all of it.