Dave,
I rebuilt the loom on my Mk3 last spring. This is an easy and satisfying task, and I recommend it over buying a finished loom. I ordered all of my wire (I stuck with the original code) and connectors from British Wire. I recall being a bit short of the power wires (blue-brown, white, and red) and had an excess of the odd stuff (horn, etc). By the way, 28 gauge is too small for your power runs. In fact, it's a bit flimsy for any use on a Norton. I'd stick to 14-16 gauge for all of the power runs (excluding starter, if that is part of your rebuild). You are using black for ground, which implies a negative-ground system. If this is the case, I would chose red for the unswitched power runs, rather than the positive-ground standard of blue-brown, to avoid confusion later. If you are NOT converting to negative ground, then staying with the standard convention of blue-brown for power and red for ground will again avoid future confusion. Switched power in either configuration is white.
You'll need extra ground wire to service the three (or more) ground points on the bike (headlight shell, rear fender at the tail light, and a large one on the frame near the battery). Connect the three points with a large ground wire or a double run of 14 or 16 gauge. I ran my loom along both sides of the upper frame tube for a smaller and neater package.
