OldIHC

How would you tell if a truck is six volts or twelve volts? I never could find a line set ticket.

As suggested here before, pull out a bulb and look at it. It's printed on it. The starter, generator, distributors have plates or IDs with voltage on them. Perhaps regulator, too.

Boy do I feel foolish right about now.

Delco color code the ID plates on starters and generators (alternators too) Black tag 6 volt red tag 12 volt orange tag 24 volt.

My early 12v IHCs have an ignition ballast resistor that the 6v systems do not. You can look for that. It's in line with the ignition coil primary side.
Also...the coil and battery have to be installed with the same polarity or the coil only puts out a fraction of its maximum output.
I used to see this problem often in the old days.

It is six volts!!!!! I got a battery and put it in made a new cable for the positive that goes straight to the bell housing. I primed the carb and the old thing fired up. Was I surprised.
I had previously changed the points and plugs and had fired it up before. That was with jumper cables everywhere. This time it was with the trucks old wiring. Nothing else works maybe because the generator and regulator are at the shop getting rebuilt.
I promptly removed the positive battery cable so the thing would not burn itself down in the middle of the night.

It's ALIVE! Now starts the longer process, good journey. Get it running, driving, stopping, then drive it a bit to figure out where you want to go from there.