Keith,
The balls should not run against the sprocket - they should press against the end of the pressure plate hub, either with or without a thrust plate. When the pressure plate lifts, it should leave the sprocket/friction plate free to rotate. When assembled, the pressure plate hub should protrude beyond the friction plate hub - if the linings are too thick, there may not be enough freedom for the clutch to free completely. If you need to fit a thrust plate, Google 'Simply Bearings' - they list various plates for thrust bearings - they are hardened, plain flat washers - the thrust plate on your clutch release cam should have a shallow track in it and that will keep the cage and balls centred on the crankshaft.
Regards,
Eddie.