Back then the manufacturers had to sell so many engines in street cars to qualify them to race. Hence Ford sold over 1,000 Mustangs with the Blue Crescent Boss 429 engine. Chevy sold lots of 427’s and Mopar 426 hemis.
i don’t recall what year they went to race only engines. It may have something to do with Toyota. Since they had to create a NASCAR engine. They didn’t and still dont build a production push rod engine. Chrysler had to create an engine too. Then Chevy created a new small block. Ford was the last to build their NASCAR specific engine. As far as I know you cannot go to a car dealer and buy a NASCAR engine in the parts Dept. From what I read race engines are leased from Roush-Yates, Hendrick and Childress and TRD. If you could buy one take it to the bank you will not get it with the camshaft they race.
As far as the cars go Ford wanted to race the Taurus. It was only made as a front wheel drive four door sedan. The rules only permitted two door cars. This is what started the made for racing stock cars from the ground up. In addition the rear wheel drive car had become passé in Detroit.
I know this simplifies the story but a book could easily be written about the evolution of the modern stock car. In 1949 it started as strictly stock but they soon found out production suspension parts couldn’t withstand the beating racing on dirt tracks around the country. Durability was a problem which in turn created safety concerns. heavy duty parts made the cars safer which also made them faster and the rest is history.