Solid axle, I removed the harness from the engine and laid it out on the garage floor. You should have two, rather large computer connectors. You start at those ends, mine were labeled and colored red and blue. I think you may have a different color, depending on the year. Get the same year harness and type when you visit the junk yard. If you don't you could end up replacing connectors at the injectors and I think some sensors are different based on year.
Since you are retaining the 4l60e, you will use a lot of the connectors I pulled out. I think you will retain most, except for the evac and second set of O2 sensors. I guess that depends on what you plan to do.
Back to the question, you will remove the split loom and remove the factory electrical wrap. Keep it all together when your pulling the wires. You don't want it to come apart and lose it's shape. Remove what you came to remove and tape it back up.
Vortec 4.8/5.3/6.0 Wiring Harness Info
is full of information for modifying your own harness. That is the site I went to for the pinout of what I needed to remove. You just need to throughly understand what your doing, just not ripping wires out of the harness and expecting it to work.
I don't know how many are still in the connector that run back to the computer, but it was plenty. It will be lot's more in your case as your retaining the transmission; I bolted it to my SM465.
My truck came equipped with dual batteries, so I have a tray on the drivers and passenger sides of the truck. The late model SUV's/trucks mount the computer on the drivers side with a little plastic tray off or near the battery tray. I went to the wrecking yard and got that little tray, welded some studs to my drivers side battery tray, and mounted the computer in basically the same location GM mounts it. When you are done modifying your harness you will have three wires coming out of it a couple of feet from the computer. You will need to hook up a relay to the switched 12v, fuse the constant (and maybe the relay) and provide a ground. There will be other wires coming from the computer, as the printout will show. Most important is the fuel pump wire. That needs run to another relay as well. I'll dig around for some pictures.