Click parts. enter your sled and look at the diagram for the driven clutch where it will tell you how many of each shim to use.
If you are using non-stock parts, I'd not know where to begin other than trying to make sure the belt was not "over the top" when the clutch was closed.
If you need some shims, let me know very soon as I recently purchased 100 of them from some guy on eBay and have about 90 extras! I'll be out of town (Michigan) over for the next few days (hopefully sledding) and back to FL on Wednesday.