Tail shaft vent?
Are you referring to that round piece if tin pressed in the backside of the yoke with a hole in the center?
The only vent that is supposed to be in this trans is on the very top of the case about halfway back.
I've welded the caps in the yokes before, as well as the pin hole in the center. Usually the yoke has a missing spline to vent any fluid back into the extension housing.
Some output shafts have a pressed on 'can' with an o-ring to seal the yoke that way.
If you totally degrease the cap and the the surrounding areas on the yoke, I bet one could use JB weld to seal it. Never tried it, but it may work.