No fastback, Mach 1, coupes, or verts got mats only the Grandé. You can buy them but they were bit factory installed, cost reductions. I always kept a thick piece of carpet in my trunk turned upside down. I never really had more that a box with a few wrenches and oil and coolant. I never went on but one trip that was to Disney World the year I got the car. I broke a driveshaft U joint in Tampa drag racing a guy. I limped home by time home it was about to fall out of the car, lol.
I do not know if it was you I told to friend Scotty Strickland on facebook of not. He has a mustang junk yard and might have the spare bracket. There was one on Ebay last week I think $100. They came in other cars not just with fold down. You got space saver with I think Magnum 500 wheels.