There might be a problem or two. I just pulled the rear leaves off an 81 F-150 and did a little reading up on them. There are three potential problems that I see in converting it to a cross leaf.
1) The perchs on late model trucks are not always centered. In other words the distance from the perch to the rear eye is longer than the distance from the perch to the front eye. This means the centering hole (which lines up the leafs in the "stack") will not end up in the center of your cross leaf setup and therefore the "stack" (how the leaves are set on top of one another) will not end up in the center of you cross member. And when you start moving the upper leaves left or right you quickly discover problem number 2.
2) The two ends of the leaves are pre bent in non- uniform bows. The bow from the perch to the rear eye is different than the bow from the perch to the front eye. I quickly tested this out with the leaves from my 81 by taking the longest leaf from the left side and trying to match it up with the longest leaf from the right side but reversing one of the leaves end for end. Sure enough, bows in the leaves do not match up. This might cause you a problem in trying to put it on a cross leaf setup. The left side will be bowed differently than the right. This is assuming you first figure out how to solve the centering problem in #1.
3) And finally, on some late model trucks the "eye"s are different on each end. I'm not sure why this or if it is true on all makes and models, but the rear eye on my 81 springs are curved in a larger diameter than the front eye (not just a mistake, its this way on both sides)
All in all, I think this conversion will cause you some problems. However, there might be a couple of solutions. What I'm doing is cutting the leaves in half and making quarter elliptical springs out of them and mounting them fore to aft parallel with the frame. But I also gave some strong consideration to cutting them in half and mounting them in the traditional cross-leaf configuration. By cutting two sets of springs in half and then just using all lefts or all rights you can eliminate all the problems of the mismatched ends and mismatched bows. You would have to figure out a little different mounting scheme then the normal setup but I think it is doable.
Dewey