It is believed that the transoms on I/O drive installations were thicker than those on V/drive installations. Here is a photo from a cut throught a 1973 31-ft Model with a single V-drive installation. This plug shows a total thickness of 1-5/8 inch.

Below is a cut-out view on a 1978 40-ft Corsair with a twin V/drive installation:

The plywood here measures 3/4" (also 5-ply as the 31-footer above) and the outer fiberglass coat is 3/8". The inner fiberglass layer is a rather thin 1/8".