[hatzie]

Since this is custom I'd draw your own color diagram for the switch plug. I'd be sorely tempted to draw up a full wiring diagram of my own for something that's Frankensteined together by someone else.

You can magnify the PDF to 200% to 300% or so, select the switch area of the picture, copy to the clipboard, paste to a image editor like GIMP or even CAD software, and print the image on a letter size sheet as a starting point.

The GM wiring diagrams will give you the switch function pinout so you can correlate your colors to the functions and move on from there.

As tempting as it is to rip it all out and start over it's usually a better idea to do bite size repairs that you can complete in a day or two and document it as you go. The '56 original wiring should be fairly simple and any original 6V wires will be oversize for 12v service.