Back in the day the letters/acronym NASCAR meant: North American STOCK Car Association.
This car was purpose built as a "stock" race car for the track, that just happens to be street legal.
One of the most iconic and classic hotrods ever built!
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Actually, you are a little off track here- If you are referring the NASCAR as the circle track boys- the models used in 1969 were from Ford the Torino, and Mercury the Cyclone, not the Mustang- see this nice quote:
"The Torino Talladegas and Cyclone Spoiler IIs are the cars that made the Dodge Charger Daytona and Plymouth Superbird necessary."
http://www.popularmechanics.com/cars/a6318/4303620/
Ford put the engine in the Boss9 to allow the engine to be "legal" in its circle track cars.
If you are referring to NHRA and the IHRA from back in the day, some of the Boss9 Mustangs did see duty on the straight line tracks
Regardless, agree, one of the most iconic cars/hot rods ever built!!