And Shelby does not use fiberglass.
Were all the body panels steel?
Thats odd... the decklid with integrated spoiler, hood, fascia etc are usually listed as being made of fiberglass.

The 1967 Shelby Mustang was available in two basic models: the GT350 carrying the high performance 289 V-8 and the GT500 hauling the larger 428ci V-8. Shelby now began parting ways from the basic Mustang model. The new 1967 Shelby model was drastically restyled to give off looks of performance, while also being able to deliver performance. A unique grill with two 7 inch driving lights could be found under the fiberglass nose extension, which was exaggerated from the production Mustang. Although most '67 Shelbys had these driving lights mounted close together in the center of the grill, some can be found with the driving lights mounted at each end of the grill to comply with some states lighting laws. Additional hood pins were added to the fiberglass hood which had a functional hood scoop. In the rear of the car, upper and lower scoops could be found, however, the lower scoops were only functional on early '67's. At the rear of the car was a fiberglass deck lid with a spoiler, as were tailights from the 1967 Cougar, without the chrome trim.
-http://www.mustangspecs.com/years/67shelby.shtml

They also mentioned this when Unique Performance was featured in Rides(shown a couple of times in Discovery Channel), Unique Performance' Shelby GT500E uses fiberglass panels just like the original Shelby GT500.

Even Eleanor in the movie didnt have functional side exhausts.. those vents were just aesthetic.
But the production version of the Shelby GT500E "Eleanor" had functional side exhaust systems.
2 1/2" Aluminum Pipe Side Exhaust System w/Tips

thats another reason why you don't see cut outs for the exhaust pipes at the rear like the original GT500.