Wiring original truck harness to LS

[C.I.Blazer]

Starting to work on the wiring phase of my LQ4 swap into my 71 Blazer. The original truck wiring is a complete mess. So many things have been modified and modified poorly (wires twisted together with tape, wire nuts, loose wires, etc.) I am trying to make sense of the wiring in the engine compartment and what i need with my standalone LS harness.

On the passenger side of the bulkhead connector, I will hook up 12PPL to the new starter and run 20W/OR/PPL (or a replacement wire since mine is trashed) to the PCM for 12v ignition power. I will keep 20T and 20DG as I am trying to use my stock temp gauge. I do not have 20DBL (have mechanical oil pressure) or 18LG and 18DG.

On the driver side of the connector, I think I can remove 16BRN as well as all the wiring to the alternator and voltage regulator since I am using the LS harness to control the new alternator. Obviously I will keep all the wires for the lights, horn, etc.

I am unclear on 12R and 18B that connect to the junction box at the battery. It looks like they both supply power to the rest of the truck system so I assume I still need them? Can I simply run 12R all the way back to the connector and remove the junctions to the alternator and voltage regulator? Do I still need the fused 18B/W that attaches at that junction?

I am posting a low res pic below, but I am sure you all already have the high res version. As always, I very much appreciate the help. I have done my best to research prior to asking this question.