For the ones I make, I make a mount using a piece of 3x4" piece of plate w/a stub welded onto it, as mentioned this allows the force from impact lifting, standing on them etc, to be transfered over a larger section of the frame, and the stub allows the sliders to be bolted on so they are removable for repairs/repaint.
Before you install them, it would be a good idea to replace the body, mounts. The rubber bushings over time degrade/collapse, which will cause the cab to sit lower, and rock enough to clank on the sliders. W/fresh body mounts you can leave yourself about an 1/8-1/4" of reveal between the sliders and the body seam.
