The gear case pressure rises and falls with temperature. How much depends on how much lube the elaborate vent (2 plugs) allowed you to get in (vs air) and the temp at the time of filling and the changes over the operational life. If OEMs weren't really concerned about oil and water mixing in that application tell me why they put 2 seals around the prop shaft, one facing in to keep the oil in and one facing out (back to back) to keep the water out???
Ever do an engine overhaul? If so, in getting the pistons into the cylinders with the rings installed you use a ring compressor to compress the rings then put the compressor against the block and tap the piston into the cylinder. lubing the Oring up good with LU oil and putting a ring compressor (any auto parts store carries them) around the Oring and then putting the bearing carrier and clamp against the LU casting and tapping should do it.
Obviously you will have to modify the ring compressor height to fit the available height on you bearing carrier....all you have to do is get it in the hole and then the compressor can come off and you tap it on in.