What size wheel, of course 15' but is it 8" wide?, if so that is a good tire choice. Not many offset's work well on these trucks. That tire will be stretched on but will work out well for you.
4" is about you can go really, after that you need to c notch the rear frame. I am 4" in back and that's about it. And if you are going to c-notch the back you should just 4 link it on coil overs or bags.