I have a strut tower brace and subs on for over 5 years I have found that the best stiffening parts I put on was jacking rails and a 4 point lower chassis brace. The jacking rails is a 3/4 square tube and a flat bar about 3/8 thick welded to the lower seam located under the rocker panels on each side of the factory seam. Those helped me a lot. I have a 91 GT and I remember that if I parked on a hump with a wheel on the hump the hatch would not close properly. After the jacking rails installed it was minimal twisting on the car. I would suggest that first. Of course a roll cage will help stiffen the car but jacking rails also help a lot. Just remember to have them welded on a flat surface (a wheel lift where all 4 wheels are on a pan) to keep the car flat like in a muffler shop.
Good Luck
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