Not necessary to pull the engine. Remove exhaust system, distributor cap, slave cylinder, starter motor, rear engine and gear box mounts, loosen front engine mount, remove the head-steady bar plus a few other obvious parts such as the gear lever gaiter and it is possible to remove the gear box from the engine without removing the engine. I have done it a few times that way. One tip is that when you go to re-install the clutch slave cylinder is to hold the circlip against the flange and then push the cylinder through the hole, then release the cir clip. Otherwise the circlip can be a real pain to fit if the cylinder is already in place.