As several have confirmed, it's a nine-inch.
Being a "Mach," it had to come from the factory with a nine-inch. Of course, over the 50 years of it's life, anything could've been swapped in.
Visually, as pointed out, the bolt-on pinion snubber is a giveaway. The eight-inch rears didn't have them. Also the socket trick on the bottom two nuts. On an eight-inch rear, you can get a socket on the bottom two nuts. On a nine-inch, no socket fits. You have to use a wrench.