I would pressure test the gearcase and only replace leaking seals.
If you look at the threads for the gear housing nut, you will see that they are full of paint, and/or worn down. Removing that nut will be really hard even with the right spanner and bearing carrier assembly puller. A lot of times you need to crack 'em with a cold chistle to get 'em out.
Reinstalling it is much worse. A dealer would rethread the gearcase. You might need to use rivets or bolts to hold it into the gearcase. That makes it hard to torque them properly.
The shift shaft seal is easier. Also, the shift shaft seal, prop shaft seal or bearing carrier seals do not often leak.
Your driveshaft bearing looks to be taper down. Push down on the driveshaft and check the play in it. If it turns smooth and has little play, replace the pump base and seal, new gaskets, wearplate and impeller and you should be good.
