Success!!
I can finally report that the grinding noise from the clutch is now gone.
I replaced both the spigot (pilot) bearing and the slave with the newly supplied slave cylinder from Alan Voigt. No more noise when clutch is depressed.
I now think that the noise was actually coming from the spigot bearing rather than the slave. I did not follow best practice of always replacing the pilot bearing when the engine is out. In fact, I didn't even give is a second look. Big mistake.
It was an oilite bush type. It had been packed with some kind of moly grease that had hardened into a dry coating that resulted in a hard, non-lubricated surface as seen below.
The basic design of the oilite bush being impregnated with oil was thereby compromised. I think greasing(incorrectly) the oilite bush probably was never a problem for the engine builder in that he typically would have his race engines regularly removed and a new spigot bearing inserted or regreased each time. But, for my road car with the bush being expected to be in situ for a long period, not the best choice.
I replaced the pilot bush with a needle bearing type installed with a sparing dab of grease. In that process I found that many of the pilot bearings provided by the usual suppliers did not include the internal seal. After several incorrect ones without a seal being supplied (Bean, Pegasus) I finally got the correct one off Ebay. Probably would work to use the unsealed one but better to use the sealed one. Sealed one on the right:
I also had a horrible time with getting the slave cylinder hydraulic lines to seal. Multiplicity of errors, many of them mine. The face of the new slave cylinder where the copper sealing washer lies was not smooth and thus did not seal. I didn't inspect it carefully before I installed assuming that it would be OK. I'll one day learn to not to assume anything regarding parts supplied. Here is the before and after prep of the sealing surface.
Further, I had leaks from my flexible bleed hose fitting - finally used an endoscope to determine this while pumping the clutch. I lost track of how many times I had the engine in and out, since you cannot readily test the concentric slave cylinder except when the engine/clutch is in place. With so much practice putting the engine in and out it became a rather easy exercise and not the dreaded chore. I found the use of a "tilter" to be especially helpful to fine adjust the angle of entry to mate with the gearbox. Ignoring all the ancillaries, I can now remove and reinstall an engine (separate from the gearbox) in very little time as a one-person job.
In the end I also chose to use stat-o-seal washers instead of solid copper ones. This particular concentric slave cylinder has the master and bleed lines connections very close to each other that limits the size of the copper sealing washers (and sealing surface) that can be used on the banjo. Tight quarters.
I will say that the issue of popping out of first gear when decelerating still exists. I had (wrongly) assumed that the bad spigot bearing might be the cause of this. Not a big deal for me. Not going to pursue this until I might otherwise be required to remove the engine/gearbox again some time in the future. I never received Voigts instructions about a gearbox modification for this issue. Another time.
Now, all of a sudden my heater control valve has decided to leak. Not a big task. But, for now the big issues of the Voigt 5 speed seem to be behind me.