Floor Pan Question

Around the rear frame rail there is a big rust hole(fist size) were the body plug was.
It is the same for both sides. You can see the floor pan on the inside.The floor pan is solid with no rust.
Is there a replacement piece for this. Looks like it goes across the whole back end and is welded to the frame rail. It almost looks like a dust cover to fill in were the floor pan bends up for rear seat. Is this part of the frame???
Is it needed for structural support.
67 cuda fastback.