Here is a short rundown of unit body cars that have factory convertibles from a closed car and some of things they changed. There may be more as I have not taken all of these completely apart.

Late 80's Pontiac Sunbird ( and other GM J bodies ) the rocker panels had an extra 2" of depth near the lower pinch weld. This didn't extend completely outward to the visible portion of the rocker so nothing looks different unless you go under the car.

80's Plymouth / Dodge / Chrysler K car. The rockers got the same treatment as the J body GM, they also got a 3" or so add on center tunnel cap, this flared out at the firewall and under rear seat creating a back bone type frame.

78 and up Ford Mustang ( Fox Body ) these got a foot square plate riveted to the torque boxes just behind the front wheels and ahead of the rear wheels. This kit was also used to stiffen up closed cars with performance motors.