Swithing the backing plates don't in increase the overall width .If thet was so you would have to change the axles to also make up the differance.Depending on the depth of the backing plates will be the width of your brake shoe and drum.B bodys came with a lot of differant size brakes. Mark
Switching A body 8 3/4 rear brakes over to B body BBP will increase the overall width some.I can't tell you how much for sure but consider the width of the brake shoe difference
