Built for the sole purpose of allowing FORD to satisfy the NASCAR homologation rule for Grand National racing.As such, Ford simply squeezed a NASCAR racing engine into the Mustang. It was a little bit of a dog until FORD added a Larger carburetor and headers. After that the car became unbeatable. The BOSS 429 was partly assembled at a ford plant and then shipped to Kar Kraft in Brighton Michigan. The 429 engines were assembled in the Lima, Ohio engine plant, and shipped to Kar Kraft where they were installed. Kar Kraft charged FORD about $4,444 to build each BOss 429 car. Once there, Kar Kraft made the suspension and body modifications, installed the 429 engine. Some of the modifications made by Kar Kraft were to move the spring towers forward 1 inch, lowering the A-arms 1 inch, flare the front fenders to make room for the F60 x 15 tires. They installed the engine, all the engine parts, trunk mounted battery, power steering cooler and oil cooler. There were 859 Boss 429 adrenalin pumping, neck snapping speed demons looking for a sucker to race and then a gas station.
On the interior the Boss 429 was equipped with the Interior Decor Group, High back bucket seats, the visibility group which included parking brake warning light, glove box lock, luggage compartment light, ash tray light and glove box light and a lighted ignition switch. The Boss also received deluxe seat belts with a warning light, a console, Two special knobs to control the manual choke and the ram air vent door. Finishing off the interior was the optional instrument gauge cluster with a tachometers.
On the exterior, the Boss had its Boss had a large BLACK (different form 1969) coordinated manually controlled hood scoop installed on the hood, a 429 logo inconspicuously placed where the mustang badge usually appeared behind the front wheel well. Colors you could buy the Boss 429 in were limited to those offered for the regular Mustang in 1970.. The front spoiler was shorter than the one used on the Boss 302 to gain clearance because the car was lowered.
FORD mated the engine to a 4 speed transmisson and a traction-loc 3.9:1 rear differential. The boss 429 engine was either the 820-T or 820-A series. They used a mechanical lifter/camshaft setup. The “A” version was fitted with limited emissions modifications.
All Bosses had the consecutive unit serial number stamped on the engine block for additional identification purposes. Boss 429’s had additional VIN numbers on the engine block, inner fenders and transmission. The Boss cars are the only mustangs that can match the original engine with the original car. Each Boss 429 received a KK sticker on the drivers door above the Ford sticker. They were numbered KK NASCAR 1201 through KK NASCAR 2059.
The Boss 429 is one of the most unique Mustangs made. Its uniqueness lies in its rarity, its engine and simply for the fact that so much effort went into modifying the basic Mustang to make the Boss 429 engine fit.
Visually, the Boss 429 sits lower than other 1969-70 SportsRoof Mustangs. And compared to the Mach 1 or Boss 302, it has an understated image. Save for the Boss 429 fender decals and large hood scoop, the Boss 429 doesn’t have any stripes, blacked-out trim, wheel or rocker panel moldings or even chrome dual exhaust tip outlets.
The Boss 429 Mustang was a limited production Mustang designed to homologate the Boss 429 engine for NASCAR racing. It would have been more logical to use the Torino, after all that was the body style used for NASCAR, but for image reasons, Ford decided on the Mustang instead.
All were built at the Kar Kraft facility in Brighton, Michigan. This was the same facility that Ford used to build the Le Mans Ford GTs. Partially completed SportsRoof Mustangs that were destined originally receive the 428SCJ engine, were modified at Kar Kraft to accept the large 429 engine. The main difficulty was getting the Boss 429 engine to fit the Mustang’s engine compartment. To do so, the shock towers were relocated outward (by hand) and while they were at it, the suspension was lowered and moved further outwards 1 inch, using spindles and control arms unique to the Boss 429. This made the Boss 429 handle much better than other big-block equipped Mustangs. Other features included Boss 429 fender decals, manually controlled hood scoop, a front spoiler that was shallower than the Boss 302 spoiler, color keyed dual racing mirrors, engine oil cooler, trunk mounted battery, close-ratio four speed manual transmission 3.91 rear axle with Traction-Lok, 3/4 inch rear sway bar (the first Mustang ever to have a rear bar), chrome 15×7 Magnum 500 wheels with F60x15 Goodyear RWL Polyglas GT tires. All Boss 429s came with the Deluxe Decor interior, 8000 rpm tachometer and AM radio.
The Boss 429 engine was based on a strengthened version of the production 429. These blocks have HP429 cast into the front of the block (driver’s side). It used four bolt mains, a forged steel crank and forged steel connecting rods. The big deal with this engine are its aluminum cylinder heads which featured a modified Hemi type combustion chamber which Ford called “crescent”. These heads used the “dry-deck” method, meaning no head gaskets were used. Each cylinder, oil passage and water passage had an individual “O” ring to seal it. The Boss 429 used a single Holley four-barrel carburetor rated at 735 CFM mounted on an aluminum intake manifold. 1969 versions used a hydraulic lifter camshaft; 1970 models got a mechanical lifter camshaft along with an improved dual exhaust system.
Each Boss 429 Mustang came with a KK sticker placed on the inside of the driver’s door above the Ford Warranty Plate which signified Kar Kraft’s production number. The first Boss 429 was numbered “KK NASCAR 1201” while the last 1969 is numbered 2059. Some Boss 429s may have this silver tape stripe missing; a small brass plate was substituted by Kar Kraft on a small number of cars.
Besides the Boss 302, the Boss 429 was the only other Mustang that had its Serial number stamped on the back side of the engine block assembly, on the inner front fender panels, on the transmission housing and on the chassis itself.
1969 Boss 429 Mustangs were available in five colors: Wimbledon White, Royal Maroon, Raven Black, Black Jade, and Candy Apple Red. 1970 versions were painted Grabber Blue, Grabber Green, Grabber Orange, Calypso Coral and Pastel Blue. All 1970 Boss 429s came with a gloss black painted hood scoop.
One would think that the Boss 429 would have been a street terror- given its impressive specifications. At best, in stock form, it equaled the 428CJs performance. The usual after market bolt-on modifications were extremely effective on waking the engine up. But as far as Ford was concerned, there was no reason to promote this Mustang- it’s only purpose for being was to get that monster engine homolugated.