A simple google search will provide, pictures, newspaper articles, and advertisements from 1947 that all include the term 'stock car' not 'stockcar'. So, /shrug.
You're continuing and I'm growing tired, so I'm going to make this simple. A complete sentence consists of a noun and a verb. Regardless of the sentence in question, car(in isolation) is a noun. A word that describes the noun is an adjective. The mistake you're making is assuming that NASCAR is an adjective, because it describes the car, to which you are incorrect. NASCAR in isolation is also a noun, because it is an association, and does not specify a specific type of car, therefor does not qualify as an adjective. At best, you can use it as a pre-position modifier, (potentially a determiner, but common day linguist no longer consider them adjectives) but unfortunately in this case, the modifier is not specific to the term in question, because there are many 'NASCAR cars', of which do not include Trucks, modifieds and whatever other specific types of classes NASCAR actually possesses. Regardless, 'NASCAR cars' is an all inclusive term, of all 'cars' included in NASCAR, like Nationwide, Sprint Cup, etc. In order to be grammatically correct, you would need to use an alternate method of dictating what type of car, the car is. The grammatically correct way, is 'Sprint Cup' Car, because that's the accurate adjective for the noun in question. So, no, you are still wrong.
Good day sir.