Routing is the first thing to look at. I don't know where the horn is on your bike, my 850 Commando had it in the stock location down by the gearbox. In that case it would not make sense for me to run a hot to it all the way from the headlight when the battery is much closer to it.
Wire nuts do a good job in wiring buildings but I would wonder about using them for vibrating machinery, although I have never seen them come apart. I am a real electrician by the way, union trained and licensed etc..
I just made my own harness for the Norton I am currently riding. I tried to make it as simple as possible with as few wires as possible. The more simple any system is the less points has that can fail.
Make every wire as short as possible and still have it loose enough so it will not fatigue, no banjo strings. You must use all stranded wire, THHN is high temp and oil and gas resistant and is very easy to find.
I had two major points on my bike that I could attach power and ground wires too. I had one power and one ground connection right by the battery, and one in the headlight. Those two areas of a British bike are convenient to anything that needs electricity.
Types of connectors that are used to join wires where vibration and heat extremes are met are often a type of split bolt called a "bug" or Stake-on style crimp on loop terminals held together by small nuts, bolts and lock washers. I had two split bolt bugs right next to my battery, one positive and one ground. Each one of them accepted a wire right from each battery terminal making them positive and negative.
Also right next to the battery I had a loop crimp connector bolted to the frame as a chassis ground with a jumper going from it directly to the bolt that holds the Lucas rectifier to the chassis. Everything that needed power at the rear of the chassis has a wire going to the negative split bolt and everything needing a ground has a wire going to the positive split bolt.
One pair of positive and negative wires runs from the split bolts up to the headlight to power everything at the front of the bike. Put a few split bolts in the headlight to join those two with whatever is up there, it does not get any more simple. When you are done you first wrap the split bolts in thick rubber tape, and THEN in standard high quality electrical tape. Before I made all the connections I put shrink tubing over all the wires and hit it with a propane torch. I did not rely on the chassis as a ground. I ran a ground wire everywhere in addition to having the chassis grounded, that way you are not going to lose your ground if a chassis bolt or part comes loose. Use red for all positive wires and black for all negative wires. For control wires such as the turn signals you can get creative with the coloring.
Any good automotive parts store will sell bullet style crimp connectors that are male and female which you can use instead of Lucas bullet connectors. Marine and heavy truck specialists may have better selections.
If you like terminal blocks then go for it, but they are much larger than a small split bolt that will take half a dozen wires, plus they have to be mounted to something where the light split bolt connector can just float where it has to. An electrical supply shop can set you up with the right size copper split bolt or "bug" for the size and number of wires you need. I am guessing the price of it will be about $1 give or take. Also tell them you want some good quality Scotch rubber tape to wrap them with and if you can find it get a roll of Scotch 88 grade electrical tape, it is their highest grade with the longest life and temperature range.
That is my industrial style, stone simple and reliable wiring harness.
Don't forget the fuse on the battery negative. I think most British bikes had about a 35 amp fuse there, which is odd because I don't think anything on the bikes draw that much current unless you are talking electric start. Also in an enclosed raceway #14 wire is usually fused at 15 amps and #12 wire at 20, so running 35 amps through either of them could burn off the insulation. I would try a 20 amp fuse on a non electric start bike and go up only if you find a problem with it, it will offer more protection than a larger fuse.
