I will use the centerline of the rear-bushing pocket as a starting point as there are differences in overall lengths of rear lower control arms between styles of rear lower control arms. The distance between the bushing pockets are constant for all rear lower control arms for GM A-Body cars between 1964 and 1972 

The sway bar hole spacing on rear lower control arms goes like this; 

The first hole center, from the centerline of the rear-bushing pocket, is located 4 and 11/16  inches from the centerline of the rear-bushing pocket. 

The second hole center, from the centerline of the rear-bushing pocket, is located 10 and 11/16  inches from the centerline of the rear-bushing pocket. 

The holes, there for, are located 6 inches apart. 

Both holes are located 1 and 1/16 inches from the top of the control arm, not the begin of the radius of the bend for the top. Lay a flat edge across the top and measure down from it to get your measurement. 

The hole size is 15/32 (between 7/16 and 1/2 inches)