Oh Sweet Noise!
It started today! After a family member who is an electrician helped me figure out the correct way to connect the batteries to the series-parallel switch it fired up and ran.
Cranked it over 3 times, about 30 seconds each time, and it fired on the third try. After a few minutes of pumping the throttle while the engine warmed up it settled into a nice 700 rpm idle, IIRC oil pressure gauge read about 80, engine temp gauge about 145.
I ran it for about 5 minutes, then shut it off. Hit the starter button and it fired back up instantly.
Family members guessed that it was last run in the mid-1980's.
I didn't try to drive it, there are still some big trees that need to be cut down, but I did put it in gear and ease the clutch out and it moved.
I have a 20 second video of the engine running, but it still hasn't forwarded from my phone. Will post it when it comes through.
Going back tomorrow afternoon to clear the trees and drive it out. Will definitely have video of that.
Thanks to all who answered my questions, posted advice or offered encouragement.
Here's a picture of the battery connections: