It seems some others are thinking you are getting a new proportioning valve. It may be because you called the drum brake distribution valve a proportioning valve. If you are not getting a proportioning valve and are simply moving the existing distribution block out from the firewall, you probably have some options. Re-route the wire if it is taking an inefficient route like through wire channel on the firewall and wire clip, and instead run it from the firewall connector in a more direct way to the distribution block, or splice in a piece of wire to lengthen it and route it any way that looks neat, or
become a subscribing member of the forum and post a want-to-buy ad in the parts section and ask for somebody to send you that wire. I didn't know there was such an extension wire like that. There are people here on the forum who have their original wire harnesses stashed and offer parts and pieces to people who need them in response to want-to-buy ads, I have experience with that.