Someone say my name?
Although it is possible to modify the harness yourself for stand-alone operation, I find its well worth it to have a third party take care of that part of my customers swaps. The company I use for ALL personal LSx and customer's LSx swaps is www.Wait4MePerformance.com. They are just a 'Mom & Pop' shop, but for the money they can't be beat IMO. Jess does the tuning and Hannah does the harnesses and usually answers the phone. I am not sure what they will charge you (I get a special pricing due to the volume of business I do with them, plus they don't do any PCM work I do my own tuning with my own software), but it should be under $400 to modify the factory GTO harness for stand-alone operation and unlock and delete the no longer needed items from the PCM. When the harness comes back, ever single wire has a label on it so you know where it goes.
You are more than welcome to drop my name when contacting them too, it prob won't be any cheaper, but it wouldn't hurt either.
As always, if you have any further questions, feel free to call me or email me.
Take care.