Hi Peter
There have been several past threads on topic of building wiring harnesses. You have already discovered one of best tips, lay out the harness on a sheet of plywood. All of the cloth cover wire with trace patterns, ends and even the braiding sources are avaliable. The cost is not bad I've made a number of CMP harness over the years and the end cost of each was less than or equel than buying a harness for modern truck of similar size.
Here are some of the past threads:
http://www.mapleleafup.net/forums/sh...ghlight=wiring
http://www.canadianmilitarypattern.c...nformation.htm
http://www.canadianmilitarypattern.c...%20Harness.htm
I have used Vintage Wiring of Maine as a source of wire and of having the cloth braided cover put on. He will sell you the wire by the foot, it is modern copper wire with vynel wire insulation with cloth cover with the correct color trace covering. The other company that I have used with good success is Rhode Island Wiring Service
http://www.riwire.com/
Good luck and keep us posted.
Cheers Phil