Yee ha! Well, I fired it up on it's own electrical system for the first time in what... more than 20 years?
"Almost" everything works. Oil pressure, tach - check.
Temperature guage? Not so much - also not sure eveything's 100% with the charging system.
Temperature gauge: Weird. Goes to below 100 degrees (limit of travel on the needle) and stays there -- until I let the motor idle down, then it creeps down towards the middle of the scale and bounces. It's acting like a tach, sort of - it moves "upward" (colder is "up" on the scale) as soon as I take it off idle and that's where it stays. I know the motor's warming up (duh) and it had to get close to 180 because the cooling fans cycled on and off (yay). I just tried completely disconnecting the lead from the sending unit, and it behaved exactly the same way. Weird.
Another weird thing. GENerator light glows faintly. When I first turn on the ignition, it's almost not on. Seems to gradually increase in intensity over the span of 20-30 seconds, engine on or off, whenever the ignition is on. Battery's new, and I keep it on a battery minder. Voltage is where it ought to be, 12.8 engine off - 14.3 to 14.8 engine running, and I see a dip when I turn on the heater blower on high, then it comes back. Voltage regulator (external) is brand new, got a good one (Echlin) from Napa. I did take the alternator in a few months back and had it tested. It showed to be good. Ground straps from block to body and from body to frame are in place and brand new.
BRAKE warning light comes on and stays on even if I press the pedal, but I'm mostly hoping that's just because I haven't finished bleeding the brake system yet so I'm not concerned - yet.
Headlight relay for ignition works great
Thanks again Brian.
Does anyone have solutions to these mysteries?