I'm considering putting after market wheels and tires on my z71 2007 (new body style) Silverado. The truck has stock 265/65/18s. I was considering going with 305/65/17s. Has anyone done anything similar? Will going from 18s to 17s cause problems? Will the 305 (12" width be a problem).

If I do decide to stay with 18s what's the largest tire I can put on with no lift? Or what's the largest tires I can put on with a 2" leveling kit?

Its gonna be close (are we talking nbs or NNBS). A lot will depend on the wheel. I ran 305/50R20's on 20x8.5 inch wheel. Had to do a little bit of trimming on the front valence to clear, when I switched to a 20x10, I had major fitting issues. I had to cranked the TB about an inch and a half to get decent clearance, and still rubbed pretty bad. To fit the tires size it seems your looking for. I would go with a wheel no wider than 9 inches, 8-8.5 would be best (and backspacing is your friend).

With a leveling kit and narrow wheels you could probably get a 33" in there comfortably. But like I said before, it all really has to do with the wheels.