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The Boss 429 is recognized as being among some of the rarest and highly valued muscle cars to date.[1] In total there were 1359 original Boss 429s made. The origin of the Boss 429 was to fulfil Ford's need to homologate the 429 semi-hemispherical engine for NASCAR racing. Ford was seeking to develop an engine that would better compete with the 426 Hemi from Chrysler in NASCAR's Grand National Division, now known as the NASCAR Cup Series. NASCAR's homologation rules required that at least 500 cars be fitted with this motor and sold to the general public. After much consideration, it was decided by Ford that the Mustang would be the car that would house this new engine.

With the release and homologation of the Dodge Charger Daytona from its rival manufacturer Dodge, the Boss 429 Mustang is solely left as a road-going version and all of its NASCAR plans were scrapped. However, the 385 series Boss 429 engine itself was used in NASCAR until 1974 in other Ford and Mercury body platforms, and later in NHRA Pro Stock class during the 1980's, its main proponent there being Bob Glidden's championship winning Pro Stock Thunderbird.

The Boss 429 engine evolved from the Ford 385 engine. It used four-bolt main caps, a forged steel crank and forged steel connecting rods. The engine featured aluminum cylinder heads, with a semi-hemispherical type combustion chamber which Ford called the "crescent". These heads used the "dry-deck" method, meaning the coolant circuit for the block was separate from the cooling circuit for the head The "dry-deck" method of design is intended to strengthen the block by removing the open coolant jackets around the cylinder and reduce hot spots by providing more direct cooling. Each cylinder, oil passage and water passage had an individual "O" ring style seal to seal it tight. Although a head gasket is used, it only seals the cylinder for combustion so a failure at one point won't compromise the others.[clarification needed]

The Boss 429 engine came standard with a single Holley 4-barrel carburetor rated at 735 CFM mounted on an aluminium intake manifold that flowed well for its time.[2] 1969 cars featured a hydraulic lifter camshaft while 1970 models got a solid lifter camshaft along with an improved dual exhaust system, though rated power output stayed the same.

The Mustang's engine compartment was not wide enough to accommodate the massive Boss 429 engine, so Ford contracted with Kar Kraft of Dearborn, Michigan, to modify 4-speed Cobra Jet Mustangs to accept it. Kar Kraft was a Ford exclusive experimental facility that functioned as Vehicle Engineering for Ford's Special Vehicles. Kar Kraft had previously developed the first GT40 MKII (winner of the 1966 24 Hours of Le Mans) and designed and built the GT40 MKIV (winner of the 1967 24 Hours of Le Mans).

Production of the Boss 429 began in 1968 at the Ford Rouge plant, where front apron assemblies revised to accept the large Boss 429 engine were installed during vehicle construction. These also provided a stronger and cleaner front structure, important considerations on such a performance car, and were coupled with reworked front fenders. The cars were then shipped to Kar Kraft's new assembly plant in Brighton, Michigan for engine installation and further modifications.

Such a large big block engine made the car nose heavy. To help offset this, the battery was relocated to the trunk, and a 3/4" sway bar was added to the rear end to limit body roll. This was the first Mustang ever fitted with a rear sway bar, giving it better handling than the other large engine Mustangs of the time, making it a much more capable track car. So did an oil cooler which allowed both high RPM and endurance loads on the engine, and a manually controlled hood scoop for bringing in denser air for more thorough combustion. For better ground clearance the front spoiler was made shallower than the Boss 302's, and a 3.91 ratio rear axle came with a "Traction-Lock" limited slip differential. Other features included, color keyed dual racing mirrors, an 8,000rpm tachometer, and AM only radio.