For 67, what gauge wire goes from the battery positive to the junction block? Also, I assume that's not the fusible link. That is elsewhere in the harness

Unfortunately I have to go off of memory as my car is still in the paint shop. That wire is brown and it comes out of the the Spring Ring battery connector along with the main positive cable. It's a decent size - somewhere around 3/8's to 1/4 inch in diameter.

The repro battery cable is really nice and that wire is included.

Is it as big a 10 gauge? Or more like 12 gauge? I'm using a generic battery cable with an added wire. I used a really wimpy wire when I originally did it and am lucky it didn't fry.

I would not go any smaller than 10 gauge and you may want to consider 8.

Went with 12. It's much heavier than what was there. There's no AC or any power stuff so there's not much to use power besides the lights , ignition, heater blower, and radio.

Don't have hands on with a 67 but should be close to the 68.
Should have a #12 to #10 wire from the alternator output splice to the junction block on the right side of the rad support then a # 14 fusible link from the junction block to the battery.
Those wires are only there to charge the battery and power some items when the engine is not running, such as the courtesy lights, maybe the radio and the ignition/starter relay. Everything else is powered from the alternator through the junction at the horn relay when the engine is running. The A/C, heater, headlights, radio, etc are being powered by the alternator while the engine is running and not from the battery. The fusible link from the junction block to the positive post will save the battery if something shorts out and draws too much from the battery and will save the alternator and other wiring if the battery sorts internally or suffers from a thermal runaway.
Not an expert just my opinion.

Hmmm. I thought the fusible link was somewhere else in the wire harness. I couldn't figure it out looking at the wiring diagrams.

Should have three under the hood. The brown one from the junction block on the rad support to the battery; an orange one from the voltage regulator to the main splice; [main splice has four red wires spliced together, one to the voltage regulator via the orange fusible link, one to the alternator output, one to the horn relay and one to the battery via the junction block]; and a black one from the horn relay to the main power up terminal at the firewall junction block via a red wire at the junction block.
Sometimes a pain in the behind to find if indeed one does burn out, but most likely the best way to protect the car from burning up from overheated wires.
The fusible link will only burn out if there is a serious short to ground or battery internal breakdown. Unlike an inline fuse it will not burn out with temporary shorts such as burning your favorite snap-on wrench by unwittingly touching a metal body part while removing the positive battery cable while the negative is still attached.
I also thought there was a fusible link in the headlight circuit but cannot be certain about that one, or any of the rest for that matter but that is what I believe to be fact.
Oh yeah, sometimes the main splice is right at the horn relay.

That looks like the 69 setup.