70 coronet 8.75 in 70 dart

I have an old a body 8.25 down the side.of my garage, it's very tired though hence.why I'm.not using it. Are the back plates interchangeable between the 8.25 and 8.75?

3 inch u bolts I have new!

Shock plates will come off the 8.25,.so no issue there either

Do you know how much either side needs to be narrowed on the 70 b body one so I can let the machine shop know.

As they have asked me to provide measurements, I have no way of cutting and welding properly at my garage.

Yes, I believe the backing plates interchange. The measurements for the spring perches are 43" from center to center and centered on the housing. If you want to narrow it, I believe the correct width is 52 5/8 flange to flange if I remember right.