voltage dropping

[76stepsidechevy]

Recently ive noticed my voltage gauge plummits if I try to use more then one thing for any extended legth of time with the worst offender being the headlights. This is a 76 k10 with a 350/465. I had the battery and alt checked at the parts store and both tested good today, the truck starts and runs good, battery stays charged ect. With the truck running and no acc. on ive got 14 volts at the battery verified with my fluke meter, turn on one thing (blower motor, wipers, lights) and the voltage drops to 13, I can run with the blower full blast using the wipers every now and then and be fine all day but try to use the lights and anything else is a joke, voltage drops to 12 Ive got dim lights, slow wipers, and a slow blower motor. Its a little better at highway speed 2500 rpm but still have to watch what I turn on.

The truck only had a ground from the battery to the alt bracket and a smaller 10 gauge wire from the alt bracket to the frame. I cleaned all those connection points and both headlight to rad support ground last week as well as added a 10 gauge ground from the battery to the rad support.

I did notice the headlight switch gets a bit warm but not hot so I was thinking maybe I should grab a new headlight and dimmer switch.

Also it seems like when the wipers are left on for a few mins they heat up and start to draw more power (voltage starts to drop)

Where do I go from here? Im afraid to start frying stuff with low voltage mainly my stereo, im sure lights and motors dont mind the low voltage but its annoying when your blower speed switch is like a dimmer switch for your headlights at night.

This truck should have a 63 amp alt in it which I think should be more then enough for everything im running. Only extras are a head unit and 400 watt amp, and a pair of 55 watt fog lamps under the bumper. battery is an older but still good optima red top.