Wheel size different front to back on 66 barracuda

tigerpaw said
Could I change the axle shafts to match the front

Yes, but you’d need new rear brakes too. Unless you keep the SBP axle flange offset, but then you have to re-drill your drums for the new pattern. And you’d have to do that every time you replace the drums.

What axle is in the car?

DoctorDiff would be your best option for axles.