To some degree, the '55/'56 hoods flexed a bit when they were new. On my '55 Courier sedan delivery, the hood was so rusty when I first got the car, the hood flex was intolerable to me. I investigated and noticed extremely weak points where the rear brace of the hood met the upper skin. I cut off the old brace and cannibalized another hood that had a structurally sound brace. I cut it off that hood, had it dunked to0 bare metal and painted the underside of it before welding it on to my good hood skin. Another notoriously weak spot on these hoods is at that same brace near the ends where the brace dips down to the spot welds. That corner gets weak from rust and actually cracks, ruining the structural integrity. So you have the spot welds coming loose and those end corners of the brace that need to be inspected.
In the case of my hood, it had both problems, but there wasn't enough good metal left to weld patches on to make it stiff again, so I replaced it.
Ever heard of anybody having to use TWO hoods to make one good one???