So, it has been a while, but some things have happened (besides a good holiday
)
I checked the shift cable with a ball from the old 5spd gearbox. I could pop it off easily by hand, so no big surprise that it would pop off the transmission. So it happens that I still had the linking rod (which goes from the right side of the transmission, over it, to the left) around. Figuring that these sockets were unlikely to have been stressed, these might be an option. I first tried one on a ball and it was not possible to separate by hand, so quite promising. Then cut a side off so I could remove the socket.
Then the socket on the shift cable was removed and the better one put in place. I did put a small screw in to prevent it from rotating, but even without I doubt it would come loose that way.
I have done some limited driving, so can't conclude yet, but hoping this will last me a good while.