The early 9 inch rear ends ('57 through '64) had the fill plug in the back of the housing. There was no plug in the case.
You can't just say that a certain amout will be fine for any 9 inch housing. Some have a full bowl cover on the back that will hold much more oil than the covers that are flat with a bulge for the ring gear. Some housings have a long tapered center and others have a small round center. These will hold different amounts of oil.