Drivers in the CSS Racing Series use highly modified versions of everyday roadgoing cars. These use a standard stock car frame, provided by Car Crusher Creative. The bodies are based off of workshop cars.
Around the time NASCAR cars became more modified, traditional stock car racing broke off from the group and kept doing what it had always done. This fictional series is one of them, and it has been active since the early eighties.

Originally posted by Wikipedia:

"True" stock car racing, which consists of only street vehicles that can be bought by general public, is sometimes now called "street stock", "pure stock", "hobby stock", "showroom stock", or "U-car" racing. In 1972, SCCA started its first street stock racing series, with a price ceiling on the cars of $3,000. Some modern street stock racing allows safety modifications done on street stock cars.