I could be wrong here, but I seem to remember reading that the sound of the engine you are hearing is not the Mustang, but actually a GT40. This assumes that everything you read on the internet is true...
A quick search reveals this text from a Mustang site (
www.ponysite.de):
"The movie audio of the Mustang at high speed is not the actual sound of the car. Apparently, the editors dubbed in the sound of a Ford GT-40 at speed. This explains the multiple upshifts and double-clutching; the GT-40 had a 5-speed non-synchro ("crashbox") transmission.
There are times in the film when we hear the actual sound of the fastback. For example, when McQueen drives the car to his apartment (before the chase) and backs the car into a parking space. Also, when he starts the car and first leaves the car wash with the mobsters in tow. And pretty much all the shots leading up to when the Charger screeches away at the intersection.
To achieve that sound, the car had a modified version of the stock 390 exhaust system, according to my research. In 1968, big block Mustangs -- and all Shelbys -- came with a transverse muffler mounted behind the rear axle. That muffler is a crossflow design, similar to the muffler on hi-po Camaros of the same era. Ahead of the muffler and the axle were a pair of resonators in the sections of pipe right under the floor. Those resonators were essentially small glasspack mufflers. You can see a diagram of this system in most Mustang mail order catalogs.
What Max Balchowski seems ot have done is simply remove the transverse muffler and run a pair of tailpipes out the back. It's essentially a straight system with two small glasspacks. I have recreated this on my replica, using 22" glasspacks from a local muffler shop, and the sound is pretty close, judging by audio recordings I've made. However, you need to keep in mind that a 289 and a 390 sound quite different. I've never really cared for the way a small block sounds with glasspacks, the exception being the early Shelby GT-350s, which used Tri-Y headers with a much throatier sound.