The cable is auto adjusting on the mustang. If the Pedal is traveling to far the the adjuster mechnism may be worn out. The adjuster mechinsm I a plastic peice at top of pedal where cable attachs and is racheting and when the teeth wear then it no longer can keep adjustment.
The adjuster is available at some aftermarket companies or the dealer.
Drivewire clutch cable w/ quadrant
Steeda add on cable adjuster