Purists enraged by the enlarged 1967 Mustang had another thing coming when Dearborn's truly large 1971 redesign appeared. Wheelbase went up an inch, overall length increased 2.1 inches, and weight ballooned by nearly 200 pounds. Under short-term Ford president Bunkie Knudsen's direction, Ford's pony car was expanded once more to make even more room up front for even more engine. New on the options list in 1971 was the 385-series big-block V-8, displacing 429 cubic inches. Advertised output for the new 429 Cobra Jet was 370 horsepower, with or without optional ram-air induction. The sporty Mach 1 carried over, again only in fastback “SportsRoof” form, but the Boss 302 and 429 didn't. They were instead followed by the Boss 351, a 330-horsepower SportsRoof built for 1971 only. Boss 351 production was 1,806. The Mach 1 remained the Mustang's flagship through 1973. Other models of note included the patriotic Sprint hardtops and SportRoofs built only for 1972, and that year's “Olympic Sprint” convertibles.
Source: Hogerty