Frank,
You will also need to undo the central 1/2" nut that retains the sprocket/shock absorber assembly. On the RH side of the gearbox, there is a round plug with 4 holes that gives access to the head of the 1/2" bolt. With the bolt withdrawn, you will see a hollow shaft in a ballrace. With a suitable drift, tap the shaft out, but leave the ballrace in place (at this stage, the ballrace cannot be removed as it fouls the kickstart gear). The final drive assembly will come away complete with a number of brass shims - make sure you fit the same number of shims on reassembly. If you still have the side cover off the 'box, and the quadrant removed, you can manually select 2 gears at once to lock the gearbox while checking that the nuts on the tail of the main and layshafts are tight (these nuts should be locked with tabwashers).
Regards,
Eddie.