The backing plate is connected to the flange via a gasket. For all practical purposes, the "flange width" I cited in the previous post and backing plate width as it pertains to to the axle width is the same.
The flange is located on the end of the rear axle tube. The measurement given (56 inches) is outside of flange on one side to outside of flange on the other side.
The width of the rear axle tube from inside of backing plate on one side to inside of backing plate on the other side of a '72 Mustang is 56 inches.
Hope this clears it up for you.
BT