Tony, most Panteras' upper rear subframes tend to spread and distort over time, developing excess negative camber. Huge, sticky rear tires tend to make this situation worse, even cracking paint on the rear fenders. The stock bay brace is almost useless in preventing all this since, along with the upper front ball joint mounts, one simply cannot tighten the bolts tight enough to prevent bumps from making the bolts move.
With a good aftermarket (or home-made) adjustable upper bay brace, the rear chassis can be preloaded enough, slowly over time, to tease the rear subframes back into stock alignment. This makes longer upper rear a-arms redundent, although most I've seen appear stronger than stock. And at least they're better than some early efforts to 'correct' excess rear camber by lathe-cutting the lower a-arm clevis-ends so the lower a-arms were pulled inward more.... while weakening the attach points to the frame.