Mo I have a few concerns on what you are planning to do. First the rub rail will need to be removed. Second this area doesn't provide a good mounting point, not much structure here. How do you plan to attach the swim platform? Third assess from the engine compartment for thru bolting is very difficult.
See the attached link, swimplatforms.com make a one level design that sits on top of he existing platform. I have seen this on a boat at my marina and it is a very nice design