FWIW, Ive never built my own harness. But Ive watched it be done. Essentially, my friend uses nylon clothesline rope to make a template.
Install the engine and components. Start running rope lengths between sensors and components, leaving about 2" extra length at each attaching point (between sensors and components). Be sure to label each rope wire like "ignition to starter" etc. Nylon rope can be melted together where wires would be soldered using a butane torch or even just a lighter.
Pull the completed rope harness off the car and lay it up on a pegboard using wire ties to fasten it down in straight lines with the rope taught. Trace the lines onto the peg board, including marking the tie down locations, and transfer the labels onto the board as well. Then cut the ropes away.
Next, rebuild the harness on the board following the traced lines as a template, tying the wires down onto the board in the same places as the rope was to keep lengths the same. Be sure to leave enough slack (remember those 2" extra lengths you left in the rope?), or put connectors in places the harness will need to be separated for installation.
Ive seen my buddy create a no non sense harness (starter, lighting, heater blower, not much else, though I would imagine its just a matter of spending more time on it to wire in additional options) in a day using this method with almost no revisions necessary once he goes to install it in the car.
