I'm not a fan of lowering trucks.....but the rear block is 1.25". So, removing that will lower the rear 1.25". Most trucks have 2"-2.5" of rake, so you probably won't end up exactly level...but you'll have less rake than you do now. I'd say try that first, and see how you like it. You could always add a small leveling kit to the front in the future if you wanted. That being said, trucks that are "exactly level" have always looked nose-high to me for some reason.
285/55/20 should fit without any trouble at all. In fact, they'll probably fit without any lift at all. If you're wanting to run BFG AT's, then I'd recommend the 275/60/20 because of the extra height. They should fit fine with a 2" level.