It's defiently crap! I have the August 1969 Car Life Magazine with the article all about the movie. Steve McQueen's car was a 390 1968 Mustang GT. Engine mods were milled heads; ignition and carb mods.
The article states that the camera car was a modified corvette with a big block and it was barely able to keep up with the Mustang and the Charger. By the way at the end of the movie the Mustang was repaired and sold for $2500.00