big volts

Have you checked for a battery off-draw yet?
When you rev it up, say to 2000rpm, what happens at the alt stud? and at the Bat +?

Think of this circuit as one long wire, with the bat+ on one end,and the alt output stud on the other end. In between are the Fuseable link, the ammeter, and the #1 splice, plus their various connections. Thats all there is to it.So whatever is at the stud, should be at the batt+, within about 4or5%.
From the #1 splice a wire goes all the way to the ballast resistor. On route it passes through the ig sw"run", and various connections, and up to but not through, the VR. Thats it.So whatever is at the #1 splice should be at the VR blue, within 4or5%. and all should be at battery voltage with the engine off/key on. And all should be at Alt output with engine running, again within 4or5%.
Forget about whats happening at the coil or ballast for a while.
Concentrate at getting alt output to the VRblue and batt+
There has got to be at least one high-resistance connection in that circuit somewhere.
BTW 12.7 volts at the battery is not a fully charged battery. In a lead acid battery, every cell should be at 2.2 volts when fully charged. Since car batteries have 6 cells thats 6x2.2 = 13.2 volts.

EDIT It seems I goofed, again. Lead-acid cells are considered fully charged at 2.1volts; so 2.1 x 6 =12.6 . I hate this part; So sorry