They sell these at O'Reilly/AutoZone:
Adjustable Thermostatic Fan Control; Economy Model; 32-248°F.
You stick the thermal probe in the radiator, and when it hits your pre-set temperature, the fan comes on. Easy.
Just set its temperature a little higher than your thermostat temperature, and keep an eye on the temps and the fan until you're sure you've got it all configured right. Not than an '86 Comanche is as sensitive to overheating as other cars.
I like to keep an eye on my voltmeter while driving with the charging system disabled. Good news: My ABS computer is the first system to fail when the voltage drops too low. When the ABS light comes on, it's time to switch the charging system back on.