For the record, the "profile" shape of the cars has to match the production vehicle - exactly. So, the rake of the windshield, length of the roofline and angle of the rear glass have to match - exactly. They have these huge cutouts of the original shape and before each race, this huge cutout is placed along the top of the cars. There can be no varience.

A few years ago, GM officially complained to Nascar that the Fords (remember the "round" Tauruses?) were too aerodynamic. Ford was winning left and right that year. I think now that they may adjust the cars so they each have the same drag co-efficient, but I'm not sure but the cars still have to maintain that shape. As far as actual parts.... I don't know.

Yes, it is a far cry from the original concept of "stock". That is why I love Rally so much (even over Formula 1). They modify the heck out of those, but they still use a rear car to make them. And, they have to have the same engine block (rules in Rally indicate no engine over 2.0 liters - that is why there are so many 2.0 liter engines on the road).