drum to KH 4 piston question

Upper and lower control arms are the same.
You leave them alone.
You change your lower ball joints, to a DISC brake lower ball joint, or drill out your existing Drum brake lower ball joint mounting bolt holes out from 1/2 inch, to 9/16 inch.
And you should be using a proportioning valve from any disc brake A body car 1967 - 1976 will be good.
And a dual reservoir disc brake master cylinder.