Rear end width?

The brakes won't make a difference on perch location. The 3" brakes do get really close to the perches on an A rear. If you tried to run the B perch location in an A, with 3" brakes, you may have some interference issues.

The B does have the perches 1" total further apart than an A 44" vs 43". I've spread the spring to get them there before. Best to move them in.