You will not need any additional flotation with the single. I would highly recommend mounting it level on top like in the first picture.
Here is a great read about a similar project. Like myself, this gentlemen needed to add weight to the rear and not floatation. I have an 8D battery against the transom to get it level. I suspect you will have the same issue.
http://simplifying.net/phpbb/viewtop...er=asc&start=0