The Boss 429 is arguably one of the rarest and most valued muscle cars to date. In total there were 859 original Boss 429s made. The origin of the Boss 429 comes about as a result of NASCAR. Ford was seeking to develop a Hemi engine that could compete with the famed 426 Hemi from Chrysler in NASCAR's Sprint Cup Series (then known as "Grand National Division"). 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.
In 1969 there were 859 Boss 429s made by Ford Motor Company. 2 of which were Cougars for Lincoln/Mercury Race Division. There were five different colors available in 1969 (Raven Black, Wimbledon White, Royal Maroon, Candyapple Red, and Black Jade) and the only color for the interior was black. The hood scoop was the same color as the car. All these cars were a manual transmission and there was no air conditioning available due to the size of the engine. Though not originally offered as an option, it is common to add a spoiler and window louvers commonly see on 1969-1970 Mach 1's and Boss 302's.
Both model years featured a toned down exterior, as compared to other Mustangs of the era (see Boss 351, Boss 302, Mach 1), in that the only external identification of the car were the Boss 429 decals on the front fenders, aft of the front tires. The rest of the car featured a very clean look that was atypical of most Mustangs that Ford had produced which could be deceiving to some people.
To show just how special these cars were, they were given special NASCAR identification that was placed on the driver's side door. Each car was given a "KK" number which stood for Kar Kraft. KK #1201 was the first Boss 429 and KK #2558 was the last Boss 429 made.
Unfortunately sales started to drop off for the 1970 Boss 429 Mustangs and with higher production costs, gas costs, and other internal Ford problems, it was decided that 1970 would be the last year of the Boss 429. After the re-introduction of the Boss 302 Mustang in 2012 and 2013 model years, rumors have started to float around that Ford may bring back either the Boss 429 or Boss 351 with the next generation Mustang starting in 2014. At this time, nothing has been announced from Ford.
In present day, these cars are highly sought after. As of 2008, auctions on eBay and at Barrett-Jackson have brought bids of over $375,000.