Fusible link???

67Dart273 said

That is an indicator that quite high current..........higher than normal......has flowed

It might be that the main ground cable to the engine is "not actually ground" anymore, either corroded inside the cable, or whatever that could cause a partial open from the battery NEG to the block

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That's what I was gonna say. The larger ground cable is no longer grounded to the block so all current is forced to go through the smaller cable. Probably melted under cranking. I've also seen negative cables that looked okay, but once you cut them apart, they were filled with the green powder that AJ is talking about.

I'd put a new ground cable on it and see if that doesn't fix things.