Here's my experience with my conv. before and after the subframe install:
I took my car to the track when I first got it. Ran TERRIBLE. Really violent wheel hop and flex.
I just had Steeda 4190 alloy subframes welded w/ the seat crossbrace in by a pretty good welder. My car is so much stiffer. Much more fun to drive too, because if I hit 2nd really hard it doesn't feel like it bounces off the ground anymore.
Go ahead and get it done and see for yourself how your car takes those bad pot holes: Much better! It'll be the best thing to you to your car. You don't want it to twist into a pretzel after hard launches.
P.S. Make sure you have a good welder too. Although you can bolt them right in on the Fox body, you still want to weld them; makes for a really STRONG combo.
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Green '94 GT conv. -- K&N filter charger, FMS 9mm wires, Accel hipo cap/rotor, 180* thermo, 14* timing, off road H-pipe in front of 2 chamber flow's, Steeda subframe connectors w/ crossbrace, and still workin' on the rest!
My 'Stang