The overall diameter of the 285/70/17 is theoretically 1.25" larger than the 285/60/18 setup, which means you would need a minimum of another 5/8" clearance between your current setup and the cab mounts to keep from rubbing. The width of the tire will remain the same, only the height will be affected. (note that tire sizes tend to be nominal and actual dimensions can vary based on brand, tread pattern, etc. so keep this in mind if you are planning on changing to an entirely different tire)
My suggestion for checking if you will have enough clearance with 5/8" increase in radius would be to cut a strip of 5/8" thick plywood (or whatever material is readily available and easy to cut) say 2-3" inches wide and long enough to span the width of the tire. Tape this block to the tire in-line with the cab mount to check for clearance between the cab mount and the block of wood. Then turn your steering wheel through the full range while checking for clearance. You could also try finding a ditch and pull into it at an angle in order to compress your suspension and repeat the same process to check for clearance at multiple ranges of suspension travel.
