Above. I usually put a washer on the bolt, that goes up through the cross member, another washer, the spring, another washer, the nut, and snug it just enough to get the cotter pin through. Might be over kill with the washers, but hey that just me........
If the spring is placed on the bottom, then any movement of the bolt/spring would wear on the crossmember hole.
With the spring on top the bolt should never move in the crossmember hole.