2008 Sierra V6 to V8 swap

I'm doing this swap for someone else. I have a gen IV 5.3, all accessories, still need to get an intake, harness, and E38 ECM, but I'm doing more investigating before I pull the 4.3 V6 out of this truck. It has a 4L60E or a 4L65E (I'll know tomorrow when it's light out). I'm trying to get a few things answered:

1) can I get an E38 ECM (the one in it currently is an E37) and program it to run the engine like a gen III swap? or do I need to have it tech 2 flashed regardless?

2) Do I need to get a harness, or can I repin the harness in the truck using pieces of a gen III harness? in other words, can I add an extra knock sensor, extra wires for coils, and swap the injector wires to eliminate the spider?

3) is there anything in particular I need to look out for or pay special attention to when I do this?

Any advice would be great. There isn't much detailed info out there, and I haven't found a good swap post yet doing a thread search.