Your quadrant may be broken, but here's some info and what you should check. As a clutch wears out, the cable gets tighter. The self-adjusting clutch mechanism is designed to give a little slack when it gets to tight and you should hear a click when it's doing it. There's basically an awl in a ratchet on the quadrant. If it's too loose, that means that either the cable has stretched ( I doubt it could stretch that much. It's made of steel), the quadrant or cable is broken, or the quadrant wasn't reset when the clutch was replaced. I really doubt that a Mustang has the original clutch after 151k miles.
Crawl under the dash with a flashlight and look at the quadrant and see where the awl is positioned. Figure out which side of the gear is the tightest end and which side is the loosest end and use your finger to disengage the ratchet to move it to the tight end. Push the pedal in slowly. It should probably click a few times on the way down. When it comes back up, it should be set right.
They always said something about pulling up on the clutch pedal with your toe and then pushing it down slowly. Maybe this will do the same thing. Try that first since it's easier. If that doesn't work, crawl under the dash like in the previous instructions.
You may just find that the awl is broken or something. If that's the case, get a new, adjustable quadrant.
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351W 89 Mustang GT Convertible