Thats a good lookin boat!
Since you are going to do yours similar to mine. I have a few tips and things I would have done differently.
1. Use the pink closed cell foam instead of the regular styrofoam. I use the regualar styrofoam boards and havn't had any trouble so far, I check mine all the time and it is still not holding water or anything, but the guys on here scared me with so many horror stories of foam getting water logged that I will be replacing it this winter with the good stuff.
2. If you are going to make the bench that you mount the seats to out of wood. Use 3/4 ply wood or maybe even 15/32 ply. I made mine out of 2x6 with a 11/32 for the top part that is hinged and 2x6's are definately over kill and very heavy. You can save some weight by using the plywood and putting an extra piece in the center for a divider/support. Plus since I used the 2x6 and its only about 8 inches with the seat and base on top my knees are a little high when I am sitting in the seat. The plywood top, seatbase, and seat added a little height but when I re-do it this winter out of plywood I am going to make it 8" high. I think the extra 2 inches will really help the comfort. Go to a dealership and sit in a couple boats. When you find one you like measure it out and build yours to match.
I really like the extra bench idea over mounting the seats directly to the existing bench. it gets your weight more in the center of the boat, and helps to balance it out a lot better Just like a bass boat/ factory Side console design like tracker, G3, Lowe, AlumaCraft, etc use.
3. If possible build it all out of aluminum. Wood is cheaper and easy to work with but if you have the skills, money, and tools I recommend you use Aluminum.
4. Dont build a deck on top of the existing back deck. Simply install some aluminum angle 3/4 of an inch below the lip of the deck along both sides and along the rear of the rear bench creating a little ledge. You then can drop in a painted or carpeted piece of 3/4 ply cut to fit and it will support your weight, hide your tank, and whatever else you put back there. I have my tie down strap, 2 stroke oil, and oil measuring jug, along with my fuel treatment stored there.