510, I'm talking about the plywood used above deck, for the cabin sides and deck. You couldn't afford to buy G10 or any other fiberglass in a thickness that gave you strength equivalent to what is there. I'm not saying wood is as strong as fiberglass, only that the existing wood is as strong as any fiberglass you would try to replace it with, partly because it is so much thicker--as is, in that application.
As to the sagging floors, that's a matter of poor design and constant humidity. The boat was not designed for that offset mast support, the original plans show a keel-stepped mast. Wood obviously was sufficient as long as it was kept dry, which it couldn't be.
I would personally never use concrete ot fill anything in the bilge, since concrete consists mainly of water and in fact it has no strength and easily crumbles if you remove the water content. Kept in the bilge, it will attract and retain water at the highest concentration, rotting out anything it touches. This is why fenceposts and traditional wrought iron fencing were always set in molten lead--never directly in concrete or other materials that retained moisture. Today, an epoxy is more likely to be used for the barrier material. Modern stuff dropped into concrete or sand is chosen to be rot-resistant, and even then, replaced in 30 years as it rots. The old old iron set in lead? Timeless.
As powder coating is not forever, and any faults will trap moisture in between the lifting coat and the aluminum...I'm not sure I'd bury powder-coated aluminum in the floor either. Any edges that compress against it should also cut through the coating. But ten years from now, we can debate how well those choices have held up.<G>
Point taken, I will not use concrete, I will just use joices fiberglassed to the hull. I will still use the aluminum plate for to support the compression post. Yes, aluminum can be attacked and have some electrolysis, but it will take a very long time to get to the point where it would be weaker then the plywood. I used some self etching primer and then several coats of paint, instead of the powder coating.
I currently have a drip-less on my propeller shaft, fixed most, if not all leaks from the hull, and only get some moisture from the hull condensation. I even have the icebox drain fixed with a pump to pump out the melted water through an outlet above the waterline at the stern, with a shutoff valve for just in case. So, I have a dry bilge. If I get any water in the bilge from working on anything, whatever the bilge pump does not remove, I vacuum out with a shopvac.
So, as long as I own the boat, corrosion to the aluminum will be close to zero, but hey, you never know. You can always remove the aluminum plate later if need be. I'm leaving floor access to do just that, I believe nothing is permanent, and therefore, should be able to be serviced.