The 1971 Boss 351 Mustang was the last of the true muscle cars of the era. It replaced both the Boss 302 and Boss 429 from the previous two years as the performance Mustang in 1971. Packing a high output version of the 351 cubic inch Cleveland V 8 under its broad, tapered hood, the Ford Mustang Boss 351 represents one of the Blue Oval s most potent performance cars. Just 1,806 were built in 1971, their lone year of production, and they remain among the hottest performers in the world of small block Fords.