The gauge is wired in and working, yay! The installation of the gauge itself is not quite complete, as evidenced by the by electrical tape:
For those who asked about wiring, all the wiring necessary for the Autometer gauge can be found behind the dash bezel at the two cigarette lighter connectors:
G for illumination +12V, W-G for illumination dimmed ground, GR for switched power, and W-B for ground.
I spliced new wires into these existing wires to go to the Autometer gauge (I used the same color wires as original, to avoid confusion). You can see the spliced wires below, heading towards the bottom of the photo:
Here are the new wires all lined up together:
I routed the wire going to the sending unit by punching a small hole through the rubber grommet around he shifter. That way I didn't have to drill any holes in the metal. See where the purple wires disappears through the rubber piece below?:
This new sending unit wire was routed inside a split loom, and zip tied to the cooling lines along the way, to keep it away from exhaust heat. Here is the termination near the sending unit:
And here is the ground wire (it's probably a temporary routing, I'll redo it later):
Then I routed the new wires through the existing holes in the back of the dash bezel (you can see the holes below, W-B and GR go through the left hole, G and W-G go through the right hole:
And connected them to the gauge.
After all this, I started thinking that I might redo the whole installation and install a T right at the sending unit for the idiot light, to avoid installing a T into the rubber lines, routing the wire along the cooling lines, and having to ground the sensor. More than likely though I'll get steered into other projects and just leave this as is

.