I'm not even sure those do a lot since there's no provision to actually bolt to the body in that location. Maybe there is a on GNX, but my 87 Cutlass just has a place for that bushing to rest, there's no cage nuts there or anything.
That part of the body acts like an accordion stretching up and down, mostly squishing down. The rear seat brace combined with the #5 bushings reduces the body squishing which flexs ths rear frame rails down, eventually leading to stress cracks.