The 1970 Ford Mustang with the 429-cubic-inch engine was the Boss 429.
Ford developed the Boss 429 Mustang primarily to homologate a race-oriented 429 V8 for NASCAR competition. The engine used aluminum cylinder heads, a semi-hemispherical combustion-chamber design and heavy-duty internal components, making it substantially different from ordinary Mustang engines.
Although the Boss 429 name is strongly associated with the 1970 model year, production began in 1969. Ford built 859 examples for 1969 and 499 for 1970, for a total of 1,358 or 1,359 cars depending on the production source’s counting convention.
The engine was so wide that Kar Kraft had to modify the Mustang’s front structure before installation. The Boss 429 was advertised at 375 horsepower, while the smaller Boss 302 was a separate model designed around Trans-Am racing rules. Mach 1 and Shelby variants also offered serious performance, but neither was the specific 429-powered Mustang.