... and you don't need to get seats from a Mach 1, the frames are all the same, simply re-cover them to match your interior. You can also source from a Cougar, but the cushions may be different. There are variations of where the seat back release lever is, as early 71s had it in the center and the mid-year change moved it to the sides, making the seat backs driver/passenger specific.
I believe the seat tracks are different in convertible so make sure before you buy. If you have the convenience group it will have the automatic seat back releases also.
I will have to go back and see where you are located. There is a Mustang junk yard in Oxford, N.C. that would have them.