Yes, early (58-59) Rancheros had 9" rears in them, have one in my 65 Mustang Coupe road race car..........not sure of the years, but I think they all had a drain plug under the center till around 60 or 61........... they also are round on the back like a 8" rear, so, you have to look good and do some checking to make sure what it is......
Mine has the factory drain plug and it sure makes it easy to change the
oil in the rear end.......and yes, they are the exact same width as the 65-66 Mustangs, even the spring perches are in the exact same location...........
I'm not sure from your pics just how many bolts hold the center section in, but I think the 9" rear has 10 bolts holding the center section to the rear end housing,just something else to check......
David