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Wes, agreed that would be the perfect repair.
However, the screw terminal from Midwest Military is only 1 foot long - with distributor screw terminal at one end and female Douglas at the other.
So I have to improvise.
Because the M38A1 #12 wire is part of the regulator harness, John suggested either a 1) soldered/shrink tube splice or 2) adding a male Douglass connector.
Of those two options, which is preferred?

Neither is a good choice in my opinion. My point in my earlier post is you have shown us wiring in an advanced state of decay. Splicing a new wire to the decayed wire is not the best choice at all. My suggestion was to remove the tape around that firewall harness and replace the entire length of that #12 wire with a new piece of the proper milspec gauge wire reusing your original connectors at each end. This would require you to only purchase the required length of the correct gauge milspec wire.

If it was me, I would splice and solder the new connector and pigtail to your existing harness, to match the original length. Shrink/heat wrap to seal.

The only question remains was the end at the distributor the only compromised bit, or was it an age related condition?

The harness end will tell you that when you begin flexing it. Brittle and dry would not be good.

Just my two cents.

Sorry Wes,
I was typing while you were.

Certainly agree. If the rest of the harness is in reasonable condition then by all means replace the whole #12 wire from end to end.

The wiring throughout the vehicle actually looks surprisingly good.
I am sad to say the that break at the distributor connector was likely caused by a rookie mistake - my rookie mistake!
When replacing the radio suppression wire, I started to unscrew the connector instead of the bolt - twisting and fraying the wire.
I felt it was important to come clean on that.
With so many broken strands in the wire it likely increased the resistance, then got hot and broke.
Thanks for the help.

For those following my engine diagnostics, no blue spark and fireworks under the hood threads, here is the update.
Replaced #12 wire, per Wes's suggestion.
I now have a blue spark at the plugs.
It starts on first attempt.
Now I can actually start driving it.
Maybe with some good use - and a little luck - the rings may re-seat.
Thanks to all that helped.