[MDPotter]

bigmoe - I can share some insight as to how we did my dad's 64 and answer a few of your questions from last night. My dad's 64 has a new American Autowire classic update harness which replaces all wiring the wiring on your truck, so don't worry about reusing anything. The autowire kit has pretty good instructions and if I remember correctly the power for the autowire harness comes from the starter and/or the LS batter tap (a red box with a cover next to the alternator). The autowire harness has its own fuse block. I'm doing a swap with a 6.0 out of an 04 Escalade and I deleted the NSS because from what I've read it's not necessary. Someone can correct me if I'm wrong, but its purpose is to tell the computer which gear the transmission is in so it won't start in neutral as the primary safety reason. It also controls the idle (will idle down when shifted from park into gear). With the autowire harness and if you have a steering column with a built in neutral switch, it will prevent the engine from starting while in gear or neutral. We retrofitted a repro 67-72 automatic column and it came with the switch. Since the autowire harness is made to work with a carbureted engine, it will have wiring that you won't need - such as the distributor wiring and VSS (vehicle speed sensor - comes from LS harness). But you can use the coolant temp sensor, oil pressure sensor, and brake warning switch wires from the autowire harness to run to your LS sensors.

The harness from his LS was reworked by Speartech (back when they were doing reworks). The rework basically involved deleting the downstream 02 sensors and other emissions, coolant level sensor, and a couple other plugs that aren't used in a swap. What Speartech provided us was a reworked harness with a fuse block and relays that look just like the setup from LT1Swap. My dad recognized that the classic update harness has a few circuits for modern upgrades such as power seats, power locks, power windows, etc. What we did was test these circuits to determine which were constant hot and which were switched hot and used those spare circuits to power the LS harness and eliminated the extra fuse block and relays.

I don't know if you're reworking your harness or not, but LT1Swap is definitely the best resource I know of. It's totally doable, even if you don't have a lot of wiring experience. I started by laying out my harness on a sheet of masonite and arranged the wiring as it lays on the engine and drilled holes in the masonite (like Ucandoit2 on youtube) and zip tied it down in a bunch of spots (not tight so you can pull wires through them, just to hold them in place). From there, I pulled all the loom and tape off. Using the pinout charts from LT1Swap, I confirmed each wire with its correlating sensor/plug to confirm it's not needed and pulled them out one by one. It is as simple as cutting all the pinks and oranges from the LS fuse block and then merging them so you don't have 20 wires to connect to your power source. After you've removed all the wires you don't need and inspected the remaining wires for damage, I would recommend testing each one to make sure there is continuity by using a tester set to continuity mode and touching one lead to the pin at the PCM connector and the other end to the pin on the corresponding plug. If you really want to go the extra mile, you can layout the harness (after removing unneeded wiring and plugs) on the engine, plug everything in, and reroute the wiring so that you can mount the PCM inside the cab to keep it out of the elements and make a cleaner engine bay. It's not necessary, but I'm in the middle of it right now and think it's worth the time.

Reworking the harness is not something you'll get done in a day. It takes time, it's very detailed, and it shouldn't be rushed. I've spent a couple hours on mine in the evenings when I have time and I've enjoyed it. An LS harness is not complicated, it's just a bunch of wires to keep straight. Take your time, label everything, and take notes of what changes you make so you can reference later if you run into any issues.