Interesting. I am trying to imagine what you are referencing...is the top layer of fiberglass connected to the cabinets etc? Or? I also have a 77 Boler
and I only had carpet on top of fiberglass. How do you know the plywood is there? I'd love to hear how you solve the soft spots! I laid underlayment and vinyl planks on mine for support, but may need to follow your lead at some point.