Spring slider mounting

I followed the advice here

http://board.moparts.org/ubbthreads/showflat.php?Number=6141863

I raised the car by the frame to where the tires just lifted and there was a little bit of clearance to the forward end of the slot. As far as centering side to side on the frame, when the PO installed the front spring boxes for the spring relocate he centered the boxes in the rail and that's how I installed the sliders. I squared them to the frame rail and spaced them evenly on each side of the rail. It must have worked as they do move freely. Hope I helped you some. Good luck.