Battery becoming very hot

Thanks Karl.

Mattax said

I can't see the black wire

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I see which one you mean now. It's really fat insulation. Hopefully its a 16 gage wire inside. That's what a fusible link is - an undersized wire with special insulation.
Original probably had a connector attaching it to the red battery feed coming from the starter relay.
Here's what it looked like for '67.