Ford built an engine so large it needed a redesigned front end just to fit under a Mustang’s hood — and only built it at all because of a NASCAR homologation rule. Fewer than 1,400 Boss 429s were ever made, each one tagged with a sequential Kar Kraft door number. Read on for how a 375-horsepower rating hid an engine capable of 600-plus, and why this remains one of the most valuable Mustangs Ford ever sold.