Yeah, I had a brain fart. Being that you have the 6.0 and rear AC, you can pretty much bet that you already have the bigger radiator. It's mainly the 4.8 Tahoes and Yukons that have the smaller radiator. Although I have heard that some 5.3's have it if it's a lower model without a towing package or rear AC.
As far as the harness, I have sold quite a few to people on here, GMFS, and on PerformanceTrucks. Many people prefer the OEM harness to an aftermarket one. Mainly because the aftermarket relays and sockets are known to melt under extreme heat. The OEM relays have had no known failures so far. I've had my harness and fans on my truck since 2009 and I have not had a single issue. Just search on Google for "Nelson fan harness melted" and you will see tons of people complain about the Nelson harnesses along with the ones from LS1 fans.com and the now discontinued Blackbear Performance harnesses. LS1fans is the guy who made them for Blackbear and they had SO many problems that they stopped selling them.