I have a brass button clutch and I can't remember where it came from.....it still slips when I really drop it but I guess the pressure plate might be nearing expiry!

Anyhow you should be fine doing the clutch with the motor still in but the alignment can sometimes be a pain in the ass. Once you disconnect and remove the tailshaft (drain gb first) and the engine is resting only on the mounts (exhaust disconnected too) it will tend to rock backward giving a decent angle to get the box up and back in place.