Any drawback to keeping the drum spindles on my 66 Valiant?

Later model disc spindles and rotors will get you the larger bolt pattern and 1/2" right hand threads both sides. Aftermarket has a tapped bushing to fit your smaller ball joint to this larger spindle hole. Factory engineers determined the smaller ball joint to be adequate, so it was years into 2nd generation A-bodies before they were replaced by the larger ball joint in all builds.