When tightening the slipper nut on, if the layshaft has any side to side play then it'll lock it all by pulling at the shaft. If the alloy shim is small for somereason, the layshaft gear could be rubbing against the inside of the tranny case. if this is the case - check for side to side play, and put that shim with the alloy sleeve thing, but if the shim is too thick, it'll put side load on the bearing and lock it for that reason, be careful...
Silly question: You haven't completely forgotten to use that alloy sleeve have you? That would liteally lock the gearbox up for the above reason, it'd just jam the gear into the inside of the tranny case.
Check the motor plate for clearance on slipper plate ofcause, this is the obvious one, but I hope so obvious you would have spotted this in 0.015seconds of it bindinging. LOL
