My 88 5.0 5sp with 67k runs out just fine. No clutch slipping or popping out of gear. Only thing that seems odd is how far up the pedal travel is until it engages. Can it be adjusted? Can't think this is normal.
Ok... This can be difficult to explain over the interwebz:
Underneath your dash, you [should] see a clutch quadrant that looks like this:
Click to make it bigger
Here, you can see the teeth on it:
Inspect yours to ensure that all of the teeth are in good shape. You also want to inspect the Pawl:
You can see it in this image that was taken by a potato:
You want to ensure that the teeth on the pawl are in good shape too, not broken.
If that is all good then do this:
Push on the clutch with your hand a couple of times and observe the quadrant rotate. Just get a good look at how it moves.
Now, without putting your fingers anywhere where they will get pinched... Put your thumb on the [bottom/flat] portion of the quadrant and push [up] as you slowly release the clutch pedal with your other hand. You want to put resistance on the quadrant so that it cannot fully spring back.
When the clutch pedal comes to stop as you slowly release, pull it toward the rear of the car slowly until you hear a [CLICK]. Pull it some more until you hear two more [CLICK] [CLICK] and leave it there.
Go test drive.
If you need more, then repeat the procedure to get another click.
If you need less, Pull the clutch pedal rearward fully while [not] restraining the quadrant, to reset and begin again.