Shimming should not be necessary... if you are "in tune" with your cruiser, pay attention to how it feels accelerating and decelerating... it will feel a little different to begin with (a little stiff, bouncy, and you might feel a little driveline vibration while the springs are still new. Take it out and drive it!!! You will have to wear the springs in!! If you wheel it,... get out there and articulate those springs! It may be a month or two before they have "settled" enough to where your shackles (assuming you still have stock) are anywhere close to the "ideal" 45 degree angle. If you are planning on running longer shackles as well, you will need to shim accordingly, to return your pinion angle to the same plane as your output shafts.
Bottom line: Wait a couple months, and see how your rig is "taking" to its new lift. Make sure your u-joints are adequately (not overly) greased, and you should have no problems.
