Youre full of shit.
The Charger R/T w/440 was undeniably quicker & faster. The stunt driver in the movie (also the actor for the mob man), Bill Hickman, had to restrain the Charger for the sake of the film.
The production team even undergripped & undersized tires on the Charger to attempt to even the score, but the Mustang still couldn’t keep up.
There has been article after article on this in many car mags. I am sure many more will come. Not to say the Mustang isn’t cool, but it didn’t perform as well as the Charger in any category. It’s sole claim to fame was that it was Steve’s chariot in the movie.