12.5" wide tire is 317.5 mm. I don't think I have seen that wide a tire on an NNBS yet with a level kit. Wonder if that would clear the UCAs?
I would look like a beast if you can pull if off. Post pics!
What width and back spacing/offset are you running?
I'm running stock 20s and 34.5 x11.5. I have a friend with a leveled silverado on 12.5s and he rubs a little but not too bad. Not sure on how different the wheel wells are from silverado to tahoe though