Why is there a two hole and a four hole lower control arm? http://www.cjponyparts.com/catalog/category/view/category/10170070/id/364/?dir=asc&order=price
The earlier Mustang control arms have 4 holes so that they can be used on either side (left or right) as the strut rod and wheel stops bolt diagonally across the arm. Not sure of any other differences.
On the early Mustang the alignment is done by moving the upper control arm in and out and stuffing shims between the UCA and shock tower...the lower control arm is fixed. The later design is done by moving the lower control arm in and out and the upper control arm stays in place.