Can someone confirm something for me whether on a '68?

When the key is turned to the START position (confirmed 12V on the start circuit), is the RUN circuit supposed to have 12v while "cranking"?

What circuit and where are you checking? Mostly I've found that you'll only be getting around 10.5 volts to the starter while cranking.

You may be talking about power to the coil. If so I believe that the run circuit would be coming though the resistor to ignition and would have the reduced voltage.

Right. But whil cranking the coil should be getting full power from another circuit. On 67’s it comes off one of the small lugs on the starter. Assume 68 is the same.

The scenario I am talking about, the starter is not even in the car, so there isn't an amp draw. I noticed when I was checking my "START" position voltage by turning the key and putting a meter on the purple wire (where I had 12v), my radio would shut off until I released the key back to the run position. It made me wonder if the switch is set up to cut voltage to the "RUN" circuit while cranking.

However after posting, I noticed that only the radio and my tach light are dropping out, my other lights do not and my voltage gauge (aftermarket installed on my console) remains at 12v. So I'm assuming that power is not cut to the complete run circuit, but I would have to trace to see where my radio and tach light are pulling power from. Those circuits seem to be dropping during crank.

That makes sense. The resistor wire is prob not energized while cranking because the starter solenoid is sending power to the coil.

Yes, the ignition circuit is on when the key is in the start position and in the run position.
As mentioned above, the ignition run wire from the switch goes through a resistor wire, full battery power to the coil during start is via the yellow wire from the starter. As soon as the key is released from the start position the yellow wire no longer provides voltage. There will still be voltage potential at the yellow wire on the starter as it is connected to the coil and the ignition wire from the key, but it dead heads at the starter when the starter is not engaged.

Terminals on the ignition switch are:
Ground, Ignition, Battery, Solenoid (starter), and Accessory.
With the key in the OFF position only the Battery terminal is HOT.
When key is in Start, the battery terminal connects to the Solenoid and Ignition terminals. All three are HOT.
When the key is in Run, the battery terminal connects to the Ignition and the Accessory terminals. All three are HOT.
When the key is in Accessory, the battery terminal connects to the Accessory terminal. Both are HOT.

Power for your headlights, horn, four ways, brake lights, dome light, etc. are independent from the ignition switch, they get power from the main splice. Only accessories such as the radio, and in your case it seems, the tach light are powered through the ignition switch. That's why if you have the radio on when the car is running and you turn the ignition off the radio also shuts off. as soon as you start the car or turn the switch to accessory the radio comes back on. Other circuits tye in to the ignition switch, such as signal lights, heater, A/C, wipers, brake warning light, etc.