The Superbird had to be built and sold to the public, per NASCAR rules during that time. The car was based off the Roadrunner, but it was modified to have much better aerodynamics. The Roadrunner was about as aerodynamic as a wall, while the Superbird sported an exotic nose cone in the front and a huge wing spoiler in the rear.
The racing version of the Superbird was basically the same as the street model, but they all got the 426 Hemi engine. That engine was available tot he public, but two less-costly engines were also sold with the Superbird. Those engines included the 440 Super Commando and the 440 Six Barrel.
One odd fact about the Superbird is that they were actually slower on the street than similarly-equipped Roadrunners. That's due to the fact that they carried some extra weight but did not get any of the aerodynamic benefits from the nose cone or spoiler until it reached speeds of at least 90 miles per hour.
The Superbird did not sell well in 1970, and dealers were left with them on the lots for as much as two years. Some had to convert them back to Roadrunners because nobody wanted to buy them as a Superbird with the wild styling. That's too bad, because the Superbird is among the most valuable of the muscle cars today. The Superbird would be discontinued after the 1971 model year. NASCAR had essentially outlawed the car by changing some rules that would have required them to carry more weight to compensate for their high powered engines.
Contrary to popular belief, the Superbird is not the car driven by David Spade in the movie Joe Dirt. The car Joe Dirt drives is the very similar looking 1970 Dodge Daytona Charger. Still, there is no chance Joe Dirt would have been able to purchase that car for just a few hundred dollars. Even in rusted out, beaten up, rat-infested condition, these winged cars demand $20,000 or more. In restored condition, they go for six figures.
If you are looking to buy a Plymouth Superbird, be prepared to get out your wallet, your checkbook, and your banker. You can sometimes find one or two listed on eBay, along with some rare parts. It's also a great place to buy Superbird models and other cool memorabilia, for those who can't afford the real thing quite yet.