Thanks for all of your contributions. I was hoping the problem was anything but the battery cables because the battery is very difficult to access as it's hung below the car and also requires removing an exhaust pipe. I finally spent the hour plus this morning pulling out the battery and all the cables (including those to a shut-off switch) and found that the positive cable to the shut-off switch had no connectivity via my VOM and zero resistance. Bingo! All the others had connectivity and very low resistance.

Since I had all the cables off, and they're at least 30 years old, I replaced them with new and now the gauges all turn on, the fan spins and the engine cranks.

In my admittedly limited experience, every electrical problem prior to this one was due to a poor ground. Shiny metal-to-metal contact is your friend.