[chumbucket]

The small squares of wood were also to prevent rot from migrating too far into surrounding wood. Back then, they didn't have the better grades of marine plywood that is available today. I redid a transom a few years ago that was curved also. It just has to be done in multiple layers of plywood, saturating each layer with epoxy resin. When completed, I can almost guarantee that the transom will outlive your ownership of the boat.
I'm not saying not to use the Seacast. It looks like a good solution. Just mor logistics to be concerned with if you are truly considering making the boat a full transom. Now you're talking molds to build up the top of the transom glass layup and so forth. I think 5200 would be much too expensive for that type of project.