Any chance they have given you a V8 clutch plate which looks identical but doesn't have the recess machined on the flywheel side for the spigot bush holder.
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Two other things to check ; When box is out check the clutch fork pivot to see if its loose in bellhousing.
;Or do you have the correct thrust bearing with the spacer?
If the pivot is loose, drill and tap a thread in the base of it and bolt it into its recess to keep it secure.
Cheers Dave