On any of the new body GM's you can put a 60 dollar 2 inch leveling kit and put on a 295/55/20 tire with ZERO trouble, iv got Nittos on mine in that size and haven't trimmed anything and haven't made any adjustments to fenders or anything, tell them just order the tires you want and if they feel like they don't want your money there's plenty of other tire shops that do
I just put in 6,500k HID kit from The Retro Fit Source that looks and performs great, their products are well built in my opinion and I also added the 5,500k LED smoked fog lights, easy OEM replacement and look great, my buddy has a 2014 AT and he has same setup but the 5,500k HID kit, we've decided the 6,500k matches the color of the fog lights dead on and looks great
I run the Nitto 295/55/20, they ride really good but they do make a little road noise as expected with a M/T the Toyo 33/12.50/20 is right at the same size, mine don't rub any and iv never cut or trimmed anything, I run the 2 inch leveling kit and have zero trouble, the best way to get the tires to last is rotate them every 3,500 miles, friend of mine gets 45-50k miles on his Nittos and that's all he ever runs, they're on a 2500 too, but he rotates them religiously
Lots of great looking trucks on here, this is my 2015 SLT, RC leveling kit, tinted windows and windshield, Retro Source 6,500k HID headlights and Retro tinted 5,500k Led fog lights, 295/55/20 Nittos, color matched GMC emblems, AMI shorty antenna, Undercover flex bed cover, in the process of doing a color matched Denali front bumper