8 3/4

Dana67Dart said
7 1/4" rear backing plates are NOT compatable with A B C or E body 8 3/4 axle / housing combos.

If you are narrowing a B C E housing and narrowing the axles you would need the backing plates off of the same or 8 1/4 as noted above.
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Not 100% true. The later 10 bolt rear cover Hybrid 7 1/4 Backing plates do work with 8 1/4, 8 3/4, Dana60. They are 5 bolt backing plates and are 10x2 1/2". Will bolt right on.