The Ford 9" is widely copied my many aftermarket drivetrain companies. However the OEM Ford Gear carrier has an
oil fill plug on the LH side of the Gear Carrier "Pumpkin" or "Chunk". This is the fill port and used also to detrmine when it was full as the lubricant flows out when full. 2.8 qts is what I have read is necessary. When constructing an axle assembly threaded ports are often welded on the top & bottom for a drain and fill point. The vent can be relocated on the axle tube and a hose attached with a filter up high. Here is a good photo of my setup, the curved hose fit pefectly:
You can just barely see the edge of the OEM fill pipe plug behind the braided hose. As described above it is used for fill in standard application, I added the two bungs top & bottom that make it easier, and allow a little higher
oil level.
