Slapper bars on a GM product with the coil springs had to be for show. What I used on my '67 El Camino, see my avatar, was a pair of premade upper control arm mounts. I don't have a picture, but I'll try and describe them. They would mount to the top control arm mounts on the differential. They had an arm that dropped down with notches that aligned with the ribs on the front of the center section of the differential and a small set screw to sort of hold them in place. The arm had a mount for the upper rear control arm and gave the arm a different angle to the differential. This would keep the differential from winding up under hard acceleration. In my case with less weight over the rear end if you went over much of a rough spot in the pavement the rear end would hop sometimes. Under normal driving you wouldn't notice anything different in the handling.