From Ford press: The 1971 Mustangs were the biggest Mustangs ever – nearly a foot longer and some 600 pounds heavier than the originals. The Boss 351, with its 351 “Cleveland” V-8 and Cobra Jet heads, debuted. The Mach 1 was available with a variety of powertrains, topped by the 370 horsepower 429 Super Cobra Jet.
In 1973, the impact of gasoline shortages, rising insurance premiums and emissions controls brought the muscle car era to a close. The 1973 model was the last original Falcon-platform Mustang, and the convertible model was discontinued.