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Definitely go with the 285/65/18 if you want to fill the wheel wells. I put 275/70/18s on my truck, and it looks nice. Not quite the 11.5" width, but it has 33.5" of meatiness.
That looks good! Look at the 1st pic and the 4th pic. Notice the difference where the front of your truck hits the rear door handle of the Avalanche. SWEET!