It's my understanding that the LS1 harness and the 5.3 harness are the same except for the injector connectors. I'm not familiar with the 5.3, but with the LS1 out of a late model fbody you have 3 plugs that have the wires that are needed to add power to the PCM. Basically it's pink wires to ignition switch on, and orange is to batt pos. You'll probably need a pinout of the PCM to trace the PCM grounds from.
The hotcam should make good power in a 5.3. It's sure has a nice lope to it! If you plan on swapping out the valve springs check into a set of LS6 springs, good to about a .560 lift and a set is about 60-70 bucks.
I'm putting a LS1/4L60e into a 41 Chevy, but I haven't modified my harness yet, but hope to get to it in a couple of months. If you want a shop modified harness ready to go PM me and I'll send you a link or 2.