The lack of weight on the suspension is letting the springs travel WAY past their normal "rebound" position. Once the cab, bed, and other components are installed, the shocks will fit. If you want to install them now, place a small bottle jack on top of the frame, wrap a chain around the axle housing and over the jack, and use the jack to compress the spring. Then bolt the shock on. Repeat the process on the other side.
Jerry