Which company heavily modified the Boss 429 Mustang for Ford?

Kar Kraft heavily modified the Boss 429 Mustang for Ford.

The Boss 429 used a huge racing-derived V8 that could not fit easily into the standard Mustang engine compartment. Ford therefore turned to Kar Kraft, its specialty engineering and assembly operation in Dearborn, Michigan. Kar Kraft reworked the Mustang’s front structure, including the front aprons and related sheet-metal areas, to create room for the wide 429 engine and its associated hardware.

The work went beyond a routine engine installation. Boss 429 bodies received structural changes during construction at Ford’s Rouge plant, then were shipped to Kar Kraft’s Brighton, Michigan, facility for engine installation and additional modifications. The cars also carried Kar Kraft identification numbers, commonly shown with a “KK” prefix.

Ford created the Boss 429 primarily to homologate its race-oriented 429 engine for NASCAR, whose rules required production examples to be sold to the public. That purpose distinguishes it from the smaller Boss 302, which was developed around the five-liter limit of SCCA Trans-Am competition.

The Boss 429 is often confused with a normal Mustang fitted with a 429. Its rarity came from the extensive factory engineering and low-volume Kar Kraft process: only 1,359 were produced for 1969 and 1970.