My 69 Firebird has fiberglass replacement floor patches of some sort under the rear seats and one in the front. They were put in with some sort of real heavy duty silicone. It wasn't my doing. Although I haven't experienced and "problems" with them I think it is a patch at best. They offer no rigidity and no real support for the floor. That being said they have been in there for over 10 years. I can't imagine why that was a better option at the time but maybe it was cheaper and easier than a whole new floor pan? Either way I look forward to the day I can right the wrong.