I found a cheap way to sure up the rear on 88 to 96 Grand Prix's. The front strut tower braces from 88-93 Cadillac De Ville will fit with slight mods. I think most of the full size gm cars from this time period have the same size brace.

First take the ends off, it is two bolts for each side. I used a Dremel with a pack of cutting wheels from Walmart. I cut squares the length of the cutting wheels around the holes, meaning the size pre-use (they get smaller as you use them). Once you are done with that take it to Autozone get a pack of washers 1/2", I believe. You need to find some that fit down inside the lip of the brackets you cut to keep it centered on the bolts of the struts. I needed two per strut bolt to get it right. They didn't lay completely flat but very close to it. Also buy 4 more bolts to go on top of the original bolts or take some off a car at the yard.

Putting the bar together, put the ends back on the bar only using the outer most holes on the bar and ends of the brackets. The brackets should have one unused hole. I did one side at a time, place the bar on top, place the washers on and then put the bolts on top. Repeat for the other side and you are done. Driving you will notice the difference right away. I have the convertible brace on the front, makes almost the same difference that the front one did when i added it. Car handles like never before, total invested $30 total and 3hours junk yard and Autozone time included.