Difference between disk brake and drum brake lwr ball joints ?

DRUM brake ball joints are mounted to the spindle with 1/2 inch diameter bolts.
DISC brake (65-72) Kelsey Hayes disc brakes use 9/16 inch diameter bolts.
Drum brake ball joints can be drill out to 9/16 inch, no problem.
There is NO difference in bolt hole spacing, between drum or disc brake ball joints.

It's were you get into 73-76 A body ball joints.
They use 5/8 inch diameter mounting bolts, either drum, or disc brakes.