I'm just in the process of replacing the 5th gear on an AGS
transmission (blown 5th) with a .745 gear ass'y.
It might be possible to do this in situ, but I think it wouuld
be pretty tricky.
It's not just that you'll have to unbolt eveything to lower
the trans. end sufficiently to remove the 5th gear housing,
you also have to remove that hex socket plug, remove
the selector shaft guts and reach into that dark, little tunnel
to lock the trans. in two gears simultaneously.
Then you'll have to have sufficient clearance to unscrew,
re-torque the retaining bolt.
I suppose it might be do-able, but I think you'd find it a bit
of a pain...easier to work with on the bench.
Just my, usually ill-informed, opinion.