There are no short cuts, you have to remove the spring. Your idea of loosening the U bolts and sliding the added leaf in will not work. You should not ruin the bushings or shackles by removing the spring. If you don't have access to a floor jack, several jack stands and a heavy duty vise you may want to consider letting a spring shop do the work. Whatever you do be careful. I assume the information you received with the AAL tells you how to remove the spring pack, add the leaf and replace the spring pack. Once the leaf is added the spring pack will be shorter from eye to eye and it may be necessary to spread the spring to get it bolted back in. You can do the spreading with some blocks of wood or a crow bar and a jack. Good luck.
Clifton