I think there were two different skidplate/crossmember designs, but the belly-pan style that you probably have has several holes and is designed to go forward/backward not only within those holes, but also to another set of nutserts on the frame. This was designed to accomodate a large variety of transmission; so many that I'm pretty sure there are waaaay more crossmember location options than there ever were transmissions. Because the frame is tapered in this area, AMC couldn't design a "universal" crossmember that would sit flush under all circumstances.
Set the crossmember up, get the transmission mount nuts started, and look at the frame. That will tell you which set of crossmember holes and nutserts you need to use to bolt the crossmember up. I've found that they don't
always line up perfectly, and I frequently find myself using a driftpin and floor jack to help manipulate the crossmember. Use lots of antiseize on these bolts. Send me a card with a gift certificate the next time you need to remove the crossmember.