I would run the 315's. IF they rub, it will be when the tire tucks HARD and it will rub the top. Thanks! even when thrashed, a black truck looks mean!

You will definitely need the 2" spacer for the rear, and it wouldn't rub in the rear, anyhow. But you gotta get those tires apart. And here's the nasty thing; There's no flares for the rear to my knowlege, and with 315's sticking out with spacers, it will throw up some SERIOUS rooster tails in the rain. I put flex flares on mine. They work but they're UUUUGLY. And I hacked myself up some mudflaps from a set of NAPA generic big rig mudflaps. That wasn't too ugly, and works well.
Another thing to consider, is that the spacer uses a (22mm?) thin wall socket, so you will need that to change a tire if you get a flat. It will look mean and keep your heavy beast from getting stuck in embarrassing places.