I've found that you can come close by taking away material, drilling holes on the heavy thicker side, I have a 1/2 drill that I spin the clutch on and look for that inner edge to hold steady at speed. Its pretty much trial and error thing with washers taped on, sharpie marks all over and your clutch bell looking like swiss cheese. Kinda feel better doing this way instead of thinning out the outter bell. Of course your clutches should be balanced as well. Makes a world of difference vibration wise, plus all that wasted energy going into your wheels instead of shaking.