4"to 41/2 what backing plates

Projectile Dart said
As stated by @Professor Fate, sounds like you just re-drilled A body axles so you would need to stick with your A body brakes and just re-drill the drums to match.
If you had bought aftermarket axles then about 99% of them require brakes that would have been originally found on original BBP cars (B/C/E bodies...)

This. Aftermarket axles are spaced out a wee bit to allow the wider brakes, and then you just rob the whole mess from most any other RWD Mopar car. Re-drilling axles means you now have a custom machined wear component: The rear drum. (granted, it'll take a lifetime to wear it out)