front wheel spacers

tried searching without any luck.

what's the thickest spacer you can run on the factory studs, would like to try and use a 1/2" if possible.

285/70/17 Nitto's on a 17*9 wheel with a 18 offset. everything clears, but the tire is really close to the upper control arm. I'd also like to get the front wheels flush with the rears, 2.5" front lift.