I have just put my engine and gearbox together using an lightened M20 Flywheel, 325 Clucth, 323 TO Bearing (which is actually used on 320's as well, but the dealer told me it was only on really early cars) and the engine has gone in to the car today, can't wait to get it running it's only done 37K Miles!:)
I have piccies comparing the IS, 325 and 323 T/O bearings if someone can host them I will post them up, they are quite different, you can see why you need the 323 bearing as it's quite a lot longer and the tabs are in different places.