Clear up something in the original trans-am cars?

I think rule stated they had to use production parts. They were pretty strict about having the race cars resemble production cars. However, the big rule was the cubic inch limit. I would say that it was a marketing tool for Ford and Chevy to have race car engines in their street cars, similar to the Hemi for Chrysler. I'm going to say probably why Mopar chose the 6 pack, was to get similar horsepower out of the street engines reliably as close as possible to the race cars, if the only requirement for the race car was to use a production block and heads.