Issue resolved. It was the ignition coil.
I took the RRC to a local specialist who conducted a thorough diagnosis, most of which was encouraging.
Compression is good.
Vacuum is good.
Seals are good.
Timing was at -4 degrees, reset to -6 degrees.
My engine is in great shape.
The problem was that the Pertronix ignition coil that I installed wasn't getting along well with the other components. I had not thought to check this because it was brand new, but looking back it was the only thing I hadn't checked.
Lessons learned (or reinforced):
1. when you're replacing a lot of parts on your truck, do one at a time and test each after installation to make sure everything works as it should. It's harder to isolate a mistake or faulty/incompatible part when you change several things at once.
2. Pertronix ignition coils may not be the best match for the Lucas ignition system.