I take it you're not planning on off-roading with that wheel choice (20" rims) since there is a tiny sidewall. I run 275/65/18 on my LR3 which is the same overall size as what you've spec'd but 18" rim and I don't have much rubbing issues. Overall, there are so many variables, it's ultimately hard to say what you'll experience, but this is mine. In the beginning yes, they'll rub, but over time things will settle in. I did flatten (with a hammer) the metal seam under the wheel arch liner in the rear right. I did not relocate my coolant lines in that same area. I also used a heat gun to reform the wheel arch liner a little to make a little more room. I used to run Johnson Rods but haven't used them in years (they live in my glove box now) as I switched to LLAMS system, but I can put my truck in access mode (lower) to enter my garage and still roll and drive into a parking spot w/o rubbing.
I'm still running the bridgestone highway tires on 20" wheels that came with the truck, and they have been surprisingly capable off road, and even still riding nice on the air suspension without airing down so as to not risk the already small sidewall. I have a set of lr3 18s ready to go once I get some tires, but in the meantime the 20s have not yet been a limiting factor.
The highway tires don't do very well in mud, but thats to be expected.