The Pantera body panels are made of thin metal. Not much structural strength to them. Overall the Pantera chassis seems to be stiff. When I jack it up, it doesn't sag like other cars. But there are four problems.
Most mid engine cars have some serious cross braces in the engine compartments. Not the Pantera. The rear of the Pantera twists and wags (like the tail of a dog). This causes cracking around the tail lights and near the rear deck lid hinges. It is my understanding that chassis stiffeners prevent the twisting and wagging, and eliminate the problems causing cracking in those two areas.
The mounting points of the upper rear control arms collapse inward. This throws off the rear camber, and makes it hard to set because it changes when the car is set on the ground. Rear camber can eventually become impossible to set because the control arms run out of room for adjustment shims. People install spreader bars and adjustable upper rear control arms to compensate. I think some sort of brace could be fabricated to stabilize the distance between the upper control arms, but it would require more than the single attachment point of the factory spreader bar.
The lower A pillar mounts are the final problem, where too much force is applied over too small of an area of thin metal, thus flexing occurs which leads to cracking. I don't think the forces transmitted through the A pillars can be reduced. I assume that means the attaching points need to be strengthened, thickened, or spread over a larger area.
My Pantera has cracking around the tail lights. I've been told the cross brace in the attached picture, although small, is effective at removing most of the twisting that caused the cracking. So I bought one. I've had my eye on the PPC rear stiffener to deal with the wagging. Its 3 foot long and attaches to the chassis under the car. Sorry no picture of that one. Precision Proformance sells something similar.
One of my Pantera's A pillars has a crack where it attaches to the cowl. I'm not aware of a "bolt-on" that can strengthen the attaching point however. This requires welding something in place to strengthen the area, and then the crack can be repaired. I guess I'm living with the crack for now.