You're gonna keep the following wires; oil pressure sender, coolant temp(the one from the side of the driver's side head) and wiper wiring. These go to the bulkhead connector on the driver's side firewall. You'll not use just about All of the wiring going to the passenger side bulkhead connector EXCEPT; the large red wiring from the positive buss bar and the A/C wiring and ground wires. There is a red wire from the buss bar that travels to the driver's side connector. Keep that. Your new harness will have wiring for the fuel system and fans if you're going to electric fans. You can eliminate the truck's fuel pump relay and wiring to the sender behind the distributor. Not sure how you'd wire the PCM speedo wire to the dash though. When I did my swap, I used a cluster from a '92-'94 truck. The Moonie cluster uses it's own DRAC system, that's hard wired into the PC board and I suppose you could figure a way around that, but for me, it was easier just to swap to a later cluster that uses a separate DRAC system. That way, you just use the PCM speedo wire and attach it to the appropriate space. If you do use a later cluster, that also requires a different style harness connector at the new cluster. The new LS harness is gonna need a constant hot wire and a switched hot wire for the PCM. The constant hot, you can get from the buss bar, and the switched hot you can get from the ignition coil you're gonna remove from the original engine.
Her's another little gotcha to keep in mind. The charging system. I think it was '03 or '04 that GM changed the way the LS system charged. There was a black box connected to the negative battery cable on those LS trucks that talked to the BCM/PCM to ""feel"" electrical requirements, causing the alternator to charge the battery at a rate of 14+ volts. If you don't incorporate this in your truck, the alternator will only charge to a constant rate of 13.7 vdc (a default). Your PCM will have the green/blue connectors. The earlier trucks using the RED/BLUE PCM didn't have this arrangement, so the alternator had its' own regulation, independent of the PCM. That little black box is called a GBCM (generator battery control module), and from what little I've learned of this, a tuner may be able to change the O/S of your PCM to an earlier software to bypass this function. But then you'll also need the earlier alternator with its' own internal regulation
That's all I have for now, but any more questions, you can ask here, or PM me
