Sorry don't know anyone who sells replacement harnesses. I had to take the harness out of 3 econolines and stretched them out on my lawn and open the harness up to make one good harness. I was forntunate at the time to get a copy of a wire schematic which I have since misplaced.
That is the best right now is to get another harness and if you can find a wire schenatic. Make one good harness.
Maybe someone else may know where to get a harness or help with a schematic to help you sort the one you have.
For what its worth, I've had really good experiences with EZ Wiring kits. As long as youre not looking for concours, stock, its a simple installation (as simple as it can be, anyway), and very inexpensive.
I would get the 12 circuit kit for the truck, wires are marked every couple of inches, you can build a mounting plate to mount the fuse block to, it really cleans up under the dash.
IF you decide to build your own, make two of them, one for a '64 van!!!