Rear end question

I have a 7 1/4 in my dart now. Building a 8 3/4 with small bolt pattern axles so I can use the existing rims. Will the backing plates work from the 7 1/4 ?
When I purchased the housing at carlisle a few years ago it came with only one backing plate. The only other ones i have are from a 70 cuda 8 3/4 with 11 inch brakes.