I now have a new "shorter" cable and the Speedo checked out fine at the shop. Not to be too negative, but I'm certain my speedometer needle will continue to bounce around as I've posted above. I guess, in retrospect, that I really can't recall it ever being steady. I'll install the new cable tonight if I get the time.
But here's a thought that's been bothering me. When I pressed the annulus back into the new bearings at the rebuild of my transmission overdrive, I distinctly remember forgetting to remove the angle drive and internal speedo gear. The manual clearly says that can damage it. How likely is it that my whole problem is that I've distorted that speedometer drive gear? (true to form, always imagining, sometimes expecting, the worst)
Also, should I be able to remove that gear and inspect it while the transmission is still in the car?