Thanks Dean!! That helps a LOT!
Turns out the piece I worked on over the weekend doesn't even go to this truck. Turns out I have a bunch of pieces then that don't go to this truck. It appears to be for a vehicle with front brakes, too. Yet ... I had holes in the frame that matched up to its mounting plates exactly. Maybe my frame is from a later truck??
Anyway, I just got back from the shop. I mounted the tranny in place just to be able to figure all this out. Turns out this is much simpler than I had thought. And when I left today, I had working brakes -- both parking and service.

And now I know where the brake light switch goes!

Also, I picked up my starter and generator from Wenger's.