First, I STRONGLY suggest that you get a set of the Jim Osborn produced Mustang Assembly Manuals for your car's year. They are not without faults, but it is better than "Charmin".
I believe the nut clip you are refering to is p/n 374251-S2, a 1/4-20 nutclip. It is common to 69 to 73 Mustangs (possibly other Fords as well) used in that spot, 2 ea in 69-70 HTs and CVs, and 4 ea in 71 to 73 (assumed 73). It is shown on page 205 of AMK's Ford Fasteners, another item to add to Santa's list.
Jim