The center to center of the threaded holes in the bracket where it attaches to the underside of the dash (7 3/4").
Mine measures 7 1/2" center to center here. I think you have the correct parts (brackets)
Other measurement is the holes in the dash just below the radio opening (8 3/4").
On my example, I have found 4 holes (each approximately 1/4" in diameter) along the bottom of the dash, below the center of the radio opening area and through the channel that supports the very bottom of the metal dash panel. The 2 outermost of these 4 holes are used to secure the radio bracket (assembly). The outermost holes measure 7 1/8" apart, this is the measurement I gave earlier in another comment.
What I have done to give you these measurements is I have ran a strip of masking tape along these holes so I can measure between them all (all 4). These are as exact as I can get you at this time.
Beginning at the hole that is closest to the ash tray, we'll call that hole#1, it is one of the holes used to bolt the bracket assembly down.
Measuring over to hole #2, is 3 9/16" away. I believe this is the "center" hole
used for cars without a console to support the radio.Measuring again from hole #1, the next hole is 6" away. I cannot remember if this hole was used at this time.
The last hole is 7 1/8" away from the first hole. This would be the 2nd hole used to bolt the bracket assembly down.
John, is it possible your dash panel has been changed or altered some how? I couldn't really find anything matching the 8 3/4" measurement directly below the radio opening on my example.
Richard