Like Luke said, as long as it isn't an 11+ the bolt pattern should be 8x6.5. Most 18" XD wheels are going to be 9" wide. It doesn't sound like the guy wants to stuff the biggest tires possible underneath it, but more just have the truck look good. In that case, I'd do a ~33" tire like a 285/65/18 Nitto Terra Grappler.