I don't have the measurements handy but 64-66 chevelle used the narrow rear early 67 does too. late 67-72 use the wide rear.
I don't know about what the break down of Buick is but 66 Olds can have a narrow or wide rear. 64-65 and some 66 use the narrow rear.
A quick check I have found is 2 fingers between the control arm and shock bracket is wide and 1 finger is narrow. Your fingers my vary
