I'd be curious if the strut brace helps or hurts with the chassis stiffness. These J convertibles, especially those made before 1993, are rather flexible in the cowl shake department. The flex is in the floorpan between the front and rear masses of the body. I don't suspect the flex ( if any ) in the front tower areas contributes much to, or has much affect on, the cowl shake issue. . Front to back frame connectors would probably be a worthwhile addition. It makes a difference if the top is up or down as well. With the top lowered the flex is more noticable. Even the integrity of the door hinge pins, hinge bushings and the striker bolt affect the body rigidity on these convertibles. Keep in mind that the top and the doors help to hold the front and rear sections of the car together.
The J body coupes are much, much more stiff structurely as I'm sure the Daytona and Laser's are. At least those without the T tops. I would think that the coupes would respond better to the strut brace. In further thought, stiffening the suspension up too far in a convertible probably acts to pass more stress and flex into the body actually worsening the issue. The suspension needs a stiff and rigid chassis to work against and a convertible by it's nature would be compromised by design in this regard.
I'm not sure what Chrysler did on the 1994 and 1995 convertible models ( and possibly the 1993's as well ) but these later models are noticably less flexible than the earlier models. The 1994 sales brochure alludes to chassis "improvements" to stiffen and reinforce the convertible's body structure. If one actually reads the copy of the brochure the noted additions such as an added "flapper" doesn't make any sense.
