Hi Mike,
I assume that you have the complete cluster fitted on the plate. Have you rotated the cluster to check that it turns freely and that all the gears can be selected?
It could be that as you slide the cluster in it moves ever so slightly out of parallel so at the point it gets tight/jams turn the cluster a bit to help everything get back in to line.
I have found that the studs are a snug fit as these have to keep the cluster in place so a bit of persuasion to fully seat is not unusual.
The only problem I found was when trying to use the earlier selector plate with the strengthening strap in the very latest cases.
The earlier cases are machined to give clearance to the strap but as the later plate is a lot thicker it had no strengthening strap and the case are not machined.
So the earlier plate strap will foul the case.
Dave.