If you look into the way the early Ford Broncos were made, there is a clamp at the axle that holds the spring. As long as you limit your travel to the point where the springs always have some tension, you should be fine. I recently made some 2" spring spacers for my Sidekick, and I made them so that they captured the springs on the outside. In the back I used 91 Crown Vic shocks, and at full extension the spring will move slightly, but it will not come out. I don't have a pic yet, but I am working on it.
Hope this helps
David