So far, so good:
I just ordered the 2013 4 gauge pack and harness from GM Parts Now.
22858071 GAUGE $212.56
92247846 WIRE $19.41
I verified through three different parts websites that these were the correct parts for the 2013. These folks seemed to have the best combination of price and shipping.
Wish me luck! They should be 2-3 day delivery.