NASCAR did have a Class for the pony cars back in the late 60s/early 70s. Not sure how long the class lasted, but they usually raced against themselves. During the driver’s strike led by Richard Petty they used them as field fillers, as many of the big names pulled out of several races. In fact Bobby Allison won a race driving a 69/70 Mustang, BUT was never credited with the win in the official NASCAR Records…go figure.
As far as NASCAR going back to “Stockcars”….never happen…to bad because Detroit will never again build such cars as the Daytona, Superbird, Talladega, Spoiler, Monte Carlo SS, and the list goes on and on…