Not to sure but just make sure the pinion bearings recieves enough oil. Try giving Tom Woods a call, they are very very helpful in answering any questions related to anything that has to do with driveshafts, u-joints, angles, slopes etc.
it depends on your current driveling angle, but ive read that 6* should be the limit of any shim, anything after that, you need to start thinking seriously about remounting the perches in the proper orientation so as to achieve proper alignment.