Is that a typo -- "1/2 inch" on transom? I woulda thunk 1 and 1/2 inch for the transom.

I would use fir, not pine, if useing construction plywood. Hopefully, that is what you bought. The fir has less resin to begin with, so it will be able to absorb more of YOUR resin than the pine. Also, for the extra buck or two, it is a helluva lot nicer to work with.

Try to get as much flotation in there as you can, even if you won't have level flotation -- you never know. :o