If you have a volt meter to measure 12.59 volts at the battery, then use that meter to determine voltage all over the truck. As you can see the test light isn't giving you enough information.
Now, when the truck is off, the voltage should read about 12.6. When you turn it on but it's not running, you are powering the ignition coil, some dash lights, the blower fan, etc so the voltage will drop a bit.
So test the voltage at the battery with the truck off. Then turn it on and bring it up to 2000 rpm. Does the voltage go up or down? If it goes down, you have an alt or wiring or reg problem. If it goes up, it's working.
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1979 4wd Mikado. G200, Gemini 250 Turbo, Clive Cam, Offenhouser Intake, JE Pistons, maXspeeding Forged Rods, Blow Thru Weber, Trooper 5spd, Intercooled