Do you mean 1964? O'Day stopped building the OSPREY around 1970. If another Osprey owner doesn't post dimensions, I do have pics that might help (no dimensions though). You may need to make some cardboard patterns and go form there.
The latest issue of "GOOD OLD BOAT" magazine (August 2014?) has a great article about restoring a 1970 O'DAY Mariner and it includes a good description of hte cabin floorboards made by her owner, I think that might be a good reference even though it won't give you the design of the Osprey floorboards. It does show a way to make suitable ones.