Selector shaft seals are going to leak no matter what you do. The seals are small square cut O-rings held by a flat plate. They are woefully inadequate for sealing. Mine started leaking within a week after replacement. If you do decide to replace them, you will need to pull the floor panels and the gear box tunnel cover. I think the top cover of the transmission has to come off and you will need to wiggle out the gear selector shafts (one at a timee) pull the shift fork off and slide a new seal on the shaft. Then, replace the shift fork, wiggle the shaft back into place and move to the next one. There are three and the gear box has to be shifted between gears to get each shaft out.
I am pretty sure that you have to do all that, but its been a while....
The seals are actually not symmetrical- read the book to make sure you put them in the
Bell housing drain plug leak is engine oil coming from the rear main seal. Not much can be done other than pulling the gearbox or engine and changing the seal.
1965 88" Diesel
1963 Mercedes 300se
1975 Volvo C303