Tony,
There wasn't a gasket listed for the joint between the clutch spigot and the crankcase. If you are going to use a gasket, make it very thin - say .1mm - with a light smear of Red Hermetite.
Regarding the clutch thrust bearing - the balls should be 7/32" dia, and you should have a 'full house' less one ball - this leaves enough clearance so that adjacent balls don't scuff against each other. When the flywheel/clutch is reassembled, you should still have about 6mm of free movement on the operating lever (in the bellhousing). Without this clearance, the thrust bearing will be continually under load, and likely to overheat - this may have been the cause of oil getting onto your clutch plate (past an overheated seal). If there is not enough clearance, check out the tufnol thrust pad in the spring plate - some bikes needed a shim behind the thrust pad - others didn't. Removing the shim may be necessary to gain the vital clearance. Also, the bronze bushes on the 6 studs through the flywheel should each have a thin spacer (about 1/16") to make the build height of the clutch assembly correct.
Bear in mind that if you fit a gasket under the clutch spigot, that will also affect the clearance at the clutch thrust.
Hope this helps,
Regards,
Eddie.