I believe that in 1971 and up the front drum was changed to use a wider bushing (not sure if the sealing ring spacing changed?), and the pump support for the drum was changed because of that. That is why when you said you had an early pump with later drum, it might be a mis-match in parts? The later pumps with the baffle can still puke fluid. I just tap the front vent hole with an NPT tap and put a plug it it, and re-locate the vent to the back of the case. That way I know a leak in the pump area is not coming from the vent.
If you have Carl Monroes' torque flight handbook, I think it covers the modification.

https://www.amazon.com/Torqueflite-727-Transmission-Handbook-HP1399/dp/1557883998