Wheel size different front to back on 66 barracuda

Sounds like someone did a front disk swap on it and left the rear alone?

Not sure what axle you have in it, but the proper way to do it would be a new set of axles and the later model 5x4.5" drum brakes. Of course if you have a 7.25" rear in there you may want to pursue a different rear axle rather than spending to upgrade the current one.