Success! While working on the distributor I saw evidence of what looked like red thread locker on several other fasteners. So I suspect that someone used red thread locker on the cam screw. As you know the red stuff is meant to be permanent. Supposedly it can be loosened by heating above 500 F, but I was afraid that applying that kind of heat could do other damage. So I decided to sacrifice the upper shaft. I pulled out the distributor, removed the sleeve, and pulled out the upper shaft with the stuck cam screw. Then I clamped the shaft in the drill press and drilled the top of the cam screw with progressively larger bits until the head separated from the screw shank. With the screw head gone, I was able to pull the aluminum magnet sleeve off undamaged. In the morning I will order a new upper shaft.
This has been a tremendous learning experience for me. Many thanks to everyone here who helped me get through this.
Brian