The violet wire (wire 30A) is a resistance wire that comes from the fuse block and powers the constant voltage regulator. Because it is a resistance wire, it is probably normal for it to get hot.
I'm looking at '69 wiring diagrams, and I see no solid black wire anywhere near the instrument cluster. Either it's creative wiring by a PO (which means you get to trace where it's supossed to go), you are not seeing the tracer color on it (clean it with lacquer thinner or equivalent), or the '70 is just different than a '69 (I know there is a LOT different between the 2 years ... this could be one of them).
I highly recommend getting a wiring diagram for a '70. I love the Pro Products ones, if you can still find them. They went out of business a few years ago, but all of their diagrams were bought by another company and reproduced.
I'm looking at '69 wiring diagrams, and I see no solid black wire anywhere near the instrument cluster. Either it's creative wiring by a PO (which means you get to trace where it's supossed to go), you are not seeing the tracer color on it (clean it with lacquer thinner or equivalent), or the '70 is just different than a '69 (I know there is a LOT different between the 2 years ... this could be one of them).
I highly recommend getting a wiring diagram for a '70. I love the Pro Products ones, if you can still find them. They went out of business a few years ago, but all of their diagrams were bought by another company and reproduced.