All that Magnum front brake stuff including spindles is a direct bolt on for A bodies both early and late.
I didnt know the spindles off the late B body magnum would fit an a body. I did swap some 79 cordoba 11.75" discs and 2&3/4" piston calipers onto my 74 A body spindles. But had to use the cordoba caliper brackets, and the longer caliper bracket bolts.
Yup. They will bolt right on. You just need the 73-76 A body disc brake upper control arms.