Front Rubbing - Tires, Suspension or Wheels?

Get the 5" backspace 18x9.5. With the 275 it may work anyway. If not, you can run a small spacer, up to .25" without any issues, to get the clearance you need. If you get the 4.5" backspace and it rubs on the quarters, there's nothing you can do about it.

I agree... get the 5". Like 72blu states, you can always add a small spacer, you cannot make the wheel deeper! Once you get the wheel bolted on find out how much spacer you need and order a set of custom made hub centric spacers. Get the ID machined to fit on your hub and the OD of the center part to fit in the wheels hub. Make sure your studs are long enough to get plenty of bite, get longer studs if you have to.

Is the center bore on these wheels designed to accept a hub ring? Hub centric is always the best way to go if you can make it happen.