Hi JD, Man that is a big dance floor Is the balsa rotten under that area? If you can drag a crowbar across the balsa and it crumbles or the glass is flaky under the balsa I think I would try to remove it, cutting back to a solid portion with a skill saw set to an 1/8" and then a little deeper as you remove the rotten area. Then if it was me I would grind the underside to clean it up and lay in a layer of glass (use epoxy). Measure resin coat and glass a piece of core of your choice. Measure and resin coat / glass some cross members - 3 should do for sure and then glass them to the stringers and the underside. She ain't going anywhere now, for rest of your life anyway.Like CB recommended, if you need more storage, now is the time to plan and execute.