Fusible link melted!

I agree that it probably wasn't your ignition switch since that just switches components on the high side (12 V). It would have to be really messed up to short a wire to ground, which is your likely problem from a frayed wire. Post a photo of your under-dash wiring. You won't scare us, we have seen some amazing junk, including all the old cars I have bought that went thru monkey owner's hands.

The problem could also be that the fusible link was just old and corroded, in which case replacing it would work. If you put in an upgraded alternator like the square-back type, that is a sure way to blow the link or worse. I agree that you should bypass your ammeter, in the engine bay. Many posts on that.