You'll have 2mm more inner clearance, and the wheels will stick out 28mm more.
According to Tire Rack you'll be adding 0.7" of tread width and 0.6" of diameter going from a 255/35/18 rear tire to 275/30/19 tire. I would be concerned with the outside rubbing, but it may work.
Zpost isn't like prison where you have to beat up someone right when you show up
. We're not like other boards.