Hi Keith,
I've just read the part of the Haynes manual that you refer to. I can see the logic in what it states but in reality you can tighten the special thin headed shouldered bolts and when fully tightened you will still be able to move the sliding plate manually in order to fit and centralise the clutch sleeve. Once you are happy that the clutch sleeve fits ok you will be able to build up the clutch centre, back plate, chainwheel, basket and bearings before fitting this as a unit.
Refer to Bill's comment about the fully enclosed chaincase. I've never had one so I can't comment on that side of it.
Good luck and let us know how you get on.
Beezageezauk.