Neil C: The plug hole referred to is in the back plates of the IH made Cast Iron differentials. The CCC and later made Cast Aluminum differentials, like you have, use a different shape rear plate and do not have that plug hole. You have to fill them through that dipstick tube. If you use 1/4 inch plastic tubing to provide a vent, you will find that filling the rearend isn't as bad as you thought.
Myron B
CCSupplyRoom
Myron B
CCSupplyRoom