Hello Robert,

The answer to your question requires more information that should be on the plans. The first thing you should consider is the strength of a good marine 1/2" plywood depending on the frame spacing of the hull is likely preferable to white cedar. And if available some 60 years ago when those plans were drawn up, would probably have been the wood of choice.

It is also important to note that fiberglass cloth is no substitute for a quality core. Generally 10 or 7 oz cloth is more about abrasion control in a boat of that size than strength. Assuming the frame spacing makes sense, which it likely does, then consider something like a good kevlar or hybrid cloth on the bottom.

If you are concerned though, the simplest solution is to use 3/4" marine ply on the bottom and 1/2" on the sides. If you are working alone, to make it easier to work with, consider ripping the plywood into planks and bead and cove them and put them on the boat using epoxy to glue them together. Bar clamps will pull them together nicely and a couple of layers of 10 oz cloth will tie it all together nicely.

Good luck, and enjoy the journey