I am just going through this right now, installing a Rebel 9 + 3 set up in my 27. I spent one night just sorting through the harness and eliminating what wires my car didn't need, like radio, a/c, heater, and third brake light. Then I coiled up the remaining wires and marked them with masking tape as to their purpose (ie, rt frt turn signal, etc)

After that was done I decided on where the fuse block would go. Dan talked me into making a hinged panel to mount it, like he did, and I am glad that I took his suggestion. The underdash area in a 27 is very limited and having it swing down is making it easier and gets it out of the way while running wires.

Once it was mounted I started by running the main 10 gauge red wire back to the starter solenoid so that I had power to the block. Then I ran the wires to the ignition switch so it was live. As I continued to run wires I would temporarily wire tie them into the bundle, using 4 inch wire ties. These are cheap and are going to be cut off as I run more and more wires into that same harness, but they keep things organized as I work.

So to answer your original question, yes, I run one wire at a time, cut it to the correct length, and then terminate it with the correct end. I am also using Painless plugs in line this time so that I can unplug the wires in the event I ever have to remove the body, rather than having to cut or unscrew the wires at their ends.

Don