Not quite sure of your description. But here are some comments:

re "...The voltage itself is healthy at about 14.5 w/o any accessories running, although it drops to about 13 when the AC is on...."

That sounds about right, but the 14.5 is a little high in summertime -- it's more like a winter reading (the charging voltage is higher in winter) -- unless it's only when you first start up. If it's warmed up, the voltage should be closer to 13.8-14.0 in summer.

re "...I put a digital multimeter in its place and found a slightly-varying voltage of + or - 0.2 volts. I haven't put it across the battery terminals yet while running...."

This last sentence is what confused me! How did you report that it's about 14.5 without accessories running and about 13 with the A/C on, without putting it across your battery terminals? Where did you take your voltage measurements, if not from the battery?

Okay, going beyond that, first, you didn't report non-running voltage, which is important to demonstrate the condition of the battery -- it should be about 12.6 if you haven't run it for a few hours (you have to give it time to equalize following running the engine).

So back to the voltage variation. I do know that your clock (if it's still running in your 26 yr old car -- my own '84's clock died a long time ago) will use some current, and the current will fluctuate as the motor winds up.
I've measured the following currents from my battery on my 1993:
12 ma (0.012 amp) steady draw (2.8 ma, 0.0028 amps, for the radio's memory presets and OBD1 memory, which you don't have in your '84).
And periodically, about once a second, there is a brief pulse of 25 ma (0.025 amp) for the winding motor on the clock.

My '84, with the clock disconnected, has a steady draw of only 0.372 ma (0.000372 amps), much less because it has no radio memory, OBD1 diagnostic memory, or clock.

I would suspect that the fluctuation you see may be caused by the motor in your clock -- although I'm a little skeptical that what should be such a miniscule draw will affect the battery as much as 0.2 volts. It could be more serious -- or perhaps the clock has developed some short in it?

But why don't you use your VOM to measure the current draw from your battery, instead of just measuring its voltage. Disconnect the negative clamp (the negative is always removed first, and reconnected last, for safety!) and put the wires of your VOM across the battery's negative termal and the negative cable's end (remember to set the VOM to an appropriate scale). Then you'll get a better idea of what's happening. And see if it compares to my cars' readings.

Good luck.