No, I intend to completely rebuild the harness. New wire, new connectors, the works. I was originally just going to splice in the wire I needed and while Ive done a lot of wire splicing in my life, a ton of soldering (was a electronic and electrical tech for all my career until I went into telecommunications) I still had concerns about that process and isolating any potential issues should they arise. So I thought it better just to put in new wire, new connectors, everything.
The TXL and GXL wire that is use (more TXL than anything) is not incredibly expensive but you cannot get all the colors that are used in the engine bay at least not in small quantities, but you can get the primary, solid colors. So my plan was to just order the colors used for power and ground; Red, Black, Orange, Yellow, etc. Then use something like white for all the signal wires. I have a cable/wire labeler at home so it will be nothing to label each wire and heat shrink it.
Most of the wiring, as I have been able to determine is in the 18awg range and I was going to step up to the 16awg wiring, which is about the max you can do with the existing terminals used in the connectors, over that then you get into the terminals and such that are not widely available. Its not much of a step but it provides a tiny bit less resistance, is a little more hardy against the heat and the environment and such.
Ive already bought a few connectors and have sources for the wiring which is all info that will be in the document.
You can certainly drop to say 20awg on the signal wiring without much issue if any, it would be the power wires that I would have concern doing that with, particularly to such things like the O2 sensors since they have high current heating elements in them.