‘75 Dart Sport /6 which rear bearing size?

My rear wheels make a humming sound while driving the car. It gets more frequent at higher speeds. Long story short, we jacked up the rear end, put it in drive and used a stethoscope-type device to verify that the rear bearings are starting to fail/are making the humming sound.

How do I figure out if I need the 7.25” size or the 8.25” size rear bearings?

Thank you!
Vehicle is oem for the rear end. ‘75 Dart Sport with /6.