The fourth and final evolution of the First Generation Ford Mustang was the 1971-1973 Ford Mustang. This new iteration of the First Generation Mustang saw the model once again grow in size, as well as gain sleeker, more angular styling. Although the Mustang's styling had been thoroughly updated, the Hardtop, Convertible, and Fastback (SportsRoof) body style options remained. These updated Mustangs offered a standard inline six engine, as well as various V8 options. Alongside the Base Model, performance variants such as the Boss 351 and Mach 1 were also offered. As emissions regulations in the US began to grow tighter after 1971, engine performance dropped across the board starting in 1972. These more stringent emissions regulations also saw the Boss 351 exit production after 1971, as well as the exit of the 429 big block engine option. Even though the 1971-1973 Mustang was built towards the end of the muscle car era, when power and performance were being hindered by emissions and safety regulations, it remains a popular model, often being converted into a Custom Car, or a Race Car. The 1971-1973 First Generation Mustang marked the end of the First Generation Mustang, with the Second Generation Ford Mustang debuting for the 1974 model year.