You'd need a clutch kit. I got a EPI 500I kit install it and it good for a set of 27 inch. I went with 26 mudzilla's because there a heavy tire for one and the diameter is almost 27 anyways.
I also know you need more power the bigger you. Short of getting a cam, exhaust system, pulling down the engine and boring out getting it stroked you need to use what you got. The dyno jet kit and k/n filter and some carb mods made lots on mind. 28's are nice but if cannot spin them full of mud you're still stuck. My 26's I can spin them no problem and with alitte rocking I'm on my way. Besides I got a winch front/back modified a 650 wrap down rear to fit and installed winch so I'm covered with mud that is!!!