Daryl
The speedo drive number and the pics posted are for an automatic.
The rear cover on a stick shift car has thrust shims behind it that hold the transmission gears in alignment. Once the transmission tunnel plate is removed to get the cover off, it wouldn't be a whole lot more work to pull the trans completly out. then you could do the work on the bench where it is easier.
Do you have the tab that is broken off?
If so we could try to weld it back on to the tail housing, It's already broken, so it couldn't hurt to try.
All the stick shift sedans up to 1968 have the same gear box.
People usually don't part them out though just sell the whole box, Besides the broken speedo drive tab is fairly common, so finding a good one might be a problem. They were usually broken from trying to pry the cable out of the housing with a screwdriver.
Call if you could use a hand getting this fixed.
Al Lieffring
66 230Sl
Jones'n for a new gas tank