clutch problems

If that stang is like later foxes, there is an 'automatic' adjuster. One would pull up on the clutch pedal to make this adjustment. But like I said, I have not worked on an '83, so I dont know what they use. The service manual might or might not (latter is more likely ) have info on this.
Or if you have an adjustable cable, tighten it up.

Good luck.

Welcome to Centerforce land. J/K- it does sound like your cable isn't tight enough. It may well be that there was nothing wrong with your old clutch, if this is why you changed it. Do you have an aftermarket linkage kit? Also, if it's been doing this for awhile it may be the synchros in the tranny, at which point a new clutch wouldn't fix the problem either.
1. Stop driving it if possible to avoid damaging the tranny further.
2. If you have stock linkage, pull up on the pedal as HISSEN said, listen for audible click. Keep doing it until it doesn't click anymore. If this doesn't help, get a billet clutch quadrant, firewall adjuster, and a new stock cable because yours is likely pooped.
3. If you already have these parts, crank out your firewall adjuster until the pedal grabs where it belongs and the gears change smoothly.
4. If these things don't help, you may have replaced one bad clutch with another one. This is extremely unlikely.
5. It isn't unlikely that your tranny is shot. If it's the one Ford put in there in '83, then it's amazing it lasted this long.

My money's on #2, and I suggest #3.
Good luck

There ya go