If you look at the other pictures in the C&D article , I believe that it's the flaring of the fenders to accommodate the over sized wheels. I'm doubting the vehicle was really "tubbed", the fenders are flared almost like a wide body kit. Wheel specs would only tell us is the car has a widened rear end housing, if they just used a a large offset, or both. To get a true idea of the vehicle you would need to know the distance from the A-pillar to the B-pillar at even given point to compare to a stock '13 Mustang.
In my previous job at Honda R&D Americas, we engineered multiple odd ball cars that we called "mules". Basically we take a previous generation car/SUV and shove in different components for durability testing. Also we did a lot of work in disguising vehicles for on road testing, depending on the angles you can make a car look very different compared to a stock bodied car.
79, never had a 79 Fox, but I've had a quite a few Fox cars, including my current 82GT Fox, a SN-95, and Gen 1 Lightning. Currently looking for a 93-Up Ranger for a twin turbo 5.0L drag race truck.
This was my 89, 11.60s @137 all motor car...
