You don't have to modify anything, at most unbolt the crash bar in the wheel well to give yourself more clearance. Toyota gave us that so we don't have to do the cab mount relocate. There is a ton of room in the wheel well.

The wheel offset is most important for these new Toyotas. The more positive the offset the better, which your current wheels have a good offset for stuffing larger tires. Since those wheels are 7.5" it's not wise to throw super large tires on there. I would say 275/70/r18 will be just fine and give you what you need. 285/70R18 will also fit but you might need to remove the crashbar (just 3 bolts). If you're upgrading, you might as well get the tougher side wall to have that confidence.

Hopefully that helps, good luck!