As the Firebird is a unibody car, there is no insignificant rust if it goes through a panel or significantly weakens it. Convertibles are even more subject to structural failure, as the factory added very little bracing to make up for the removal of the roof structure.

Windshield frames are subject to rust along the header and in the glass channels. Without early attention, this rust grows down the pillars. Sometimes it is visible at the edge of the steel upper dash at the base of the windshield as a bit of blistering, sometimes visible with the cowl grille off, sometimes it's visible as rust up under the dash, or in the top of the door jambs. I've seen them rust bad enough that you could very literally fold the windshield frame down onto the dash with as little as 50 pounds of force, yet still look fine. Observe very carefully when latching the top, preferably with your hand on the windshield frame. If a power top, unlatch and release it, and retract the top. Try to flex it as if pulling yourself up from the windshield header. Moderate force only...then if power top, cycle the top back up with one hand on the windshield side channel. When the top reaches the windshield you should feel it, but the windshield frame should not move. When latching the top, the frame should not flex.

X brace under the belly is critical. My 'bird has had subframe connectors on it for 25 years, and although not factory I am glad they are there as they have provided additional stiffening, making for better ride and handling and less damage to the car.