Can I use 7 1/4" drum backing plates for 8 3/4" housing?

As long as you have the parts in hand you might as well see if they bolt up. That seems to be the way a lot of us have found how all this stuff goes together. Chrysler probably used the same backing plates based on drum size for 7.25" and 8.75" stuff. Let us know.

except the A-body specific stuff?

R.

I thought the 7 1/4 plates wouldn't bolt up to a 8 1/4 or 8 3/4" rear?

the 7&1/4 & (A body) 8&3/4 both use 1&3/4" wide shoes & as you know the shoe width has to match the drum width & requires a backing plate offset matched to them but I was thinking the 8&3/4 outer seal retainer & therefore the housing flange had 5 studs/nuts & only 4 on the 7&1/4 but I ain't sure on that. something was diferent iirc & man I was just over there earlier today but had other business on my mind

The '73 and up BBP 10" brakes off an A body 7-1/4 will bolt right on an 8-3/4 as long as the bearing to flange spacing is correct.

I found out B-Body backing plates are different than A-Body after I got a set back from a powder coater and he sent me the wrong plates back. Different width shoes as said above, the areas where the shoes ride is higher on A-Body plates.