Hey Don, thanks again. Can you help me understand what you mean by "so the glass won't draw in? and "a pattern on the outside of the body? Forgive my ignorance.
Wood an glass have different rates of expansion. If you glass the wood to the body, it may lead to a "shadow" of the edge of the floor pan visible on the outside of the car.

Pre-fabbed plywood would be a pre-cut floor made for your car. However, unless you know who made the body, I'm not sure how you would find the correct one. Cutting a floor to fit is not difficult - but marine plywood is a good idea. Painting it with a marine polyurethane varnish is a good idea also.