I put in a set of 5.5" springs and had the same problem. After wrestling with it for awhile I realized that the radius arms were not coming down far enough, so I unbolted them and pulled them out of the bushings. That let them come down enough to get the springs in. For 3.5" springs, you might not have to take them all the way out, but rather loosen them way up just to get as much bind out of them as you can. Anyway, after that, I bolted everything else up, chocked the front wheels in front and in back (of the front tires) and had a friend slowly lower the jack to let all the weight compress the springs while I made sure the radius arms went back through their bushings. Take it slow. It should work for 3.5" springs as well.