I dont know for sure just a guess, But I think aerodynamics play a big part in thier non existance in pro stock. The bodys have to retain the factory roof line and the front ends cant be altered very much. And the mustangs are too flat nosed to plow air all the way down the track and be competitive. And to keep up in a tight field that is ussually within a few thousands of a second to qualify, aerodynamics are a huge part of pro stock.