Can't tell from that photo what you mean, but all '55/56 Ford front floor braces on both side of transmission hump kinda sag in the middle and slope upward toward the trans hump. That's not any kind of damage. The floor braces are a much heavier (thicker) steel than the 18 gage floor.
People who buy the cheapo replacement floor pans typically have some trouble getting the new panel to sit down onto the floor brace channel flanges due to the curvature (sag) of the existing floor brace.
The way I always did them, I drilled holes for sheet-metal screws and screwed the new floor pan down to the channel flange. Then I welded the floor pan to the flange. Then I removed the screws and filled in the holes with mig-welding.