I finally got around to picking up a low-temp switch from an '87-89 4.9L fuel rail & wiring it into the low-speed relay ground. It works - just as the thermostat opens, the fans come on.
I still have to find a hi-temp switch to trigger hi-speed, but I did add some wiring & switches to the dash so I can force the fans off, on hi (also with A/C), or leave them to run automatically.
See also:
.
.
.
.
