Clutch symptoms...
 

Grinding when pressing the pedal down yesterday (more like pedal motion felt non-smooth when the motor was running). Grinding stopped, replaced with an annoying squeeeeeal when the pedal is pressed and a bit of a whirring even without the pedal pressed. Is this the throwout bearing?

Yes.

Concur.

You still have some time to make repair, before it pukes.

I was once on a trip when this started. It was 250 miles to the house.

The clutch held together, but by the time the trip was over it was totally shot.

Yeah the throw out bearing siezes and then spins against the fingers of the pressure plate. The fingers are about 1/8" thick and if a bit on the old side could be half worn away. Presto you are swiftly worn through one or more finger and then you will not be able to disengage the clutch at all.

So your time is pretty limited.

^^ adding to the chorus. Don't push your luck. The "good" news (since, IIRC you had a lot of hydraulic problem earlier) is that you should be able to unbolt the slave from the bellhousing and not have to screw with that part of the transmission.

From your description it certainly sounds like the to bearing and possibly the pilot bearing or bushing.

That reminds me....

Since this was about 40 years ago that I went through this I had forgotten about the solid connection from a 'melted' pressure plate. Towards the end I was shifting by changing the RPM's since I did not have a clutch to use. (Shifting without using the clutch was an old truck driver trick. If done right you don't need the clutch to shift and you extend the life of the throwout bearing and pressure plate. These items were not that expensive, but the down time on your truck sure was.)

And I had to have it towed to the shop, something I have done only twice over the last fifty years.

Yup. Throwup bearing.