I assume you're probably going to replace the floor and the underlying brace. As bad as your floor looks perhaps you could pull the head of the bold and the hex washer under it through the floor and support under it. When I replaced the same on my sedan, also a 57, I ground the off the head of the bolt but then they lower portion of the bolt was still held in place by an internal nut into a washer above the rubber body supports. It was a real PITA. I put a punch on the bolt and drove the remaining bolt through the washers above frame support.