Front wheel well sound deadener would not have differed based on model type so Mach I, GT or just a standard Mustang (now Cougars do appear to have gotten more in general but that's another thread) would have gotten the same treatment in that area. The under body sound deadener applied on Boss 302's, Grande's and Mach I's is a different subject and unrelated from the examples I've seen. The variable was the individual worker and how much they wanted to cover and where.
Looking at other cars from your plant and time period I found the following. Note that some of the sections have fallen off over the years but those are easy to spot. To understand the spray pattern consider that the product was applied from a pit or area where the worker stood below the car shooting upward
All taken from standard base models of the plant and period - Hope this helps
Looking at the patterns on the inner fender panels and the cowl sections it apparent that sound deadener extended onto the splash shield at the front and the back. Though its difficult to take and show a 3 dimensional area in a picture I would like to offer that many examples have sound deadener sprayed connecting the front and rear patterns connected on the bottom side of the fender (the bottom surface of the fender) Sometimes this connection is only approx 6" wide while other times its wider than 14"
