Mid Perch is easy and cheap/free if you DIY. It will definitely help with clearance. I'm running 17x8.5 wheels with a 0 offset and my tires stick out about 1/2" from the outside of the fender flare. Your calculations are correct that the wheel you're looking at will stick out about 3" from the truck. Personally I think that is too much and the truck would look a little too much like a roller skate. I'd stick with a 9" wide wheel with a +1 offset or at the VERY most, a 10" wide wheel with a -12 offset. I think even that would be pushing it. But if you're not going off-road, I think you'll be fine. i know at full suspension compression, I had some contact even with my relatively conservative offsets when jumping the truck at Raptoberfest.