Just from looking at it, I'd say definently put some sort of support back in it. The middle bench seat helps steady the middl/upper sides along with those support brackets on top of the seats.
So, with that covered, how do you still create the needed support? Here's a couple ideas:
1. Make a walk through bench seat. Build a bench that sits in the stock seat mounts and has the center 18" or so removed. This works ok but still leaves somewhat of a "seat" in that area.
2. Build small storage compartments that contour the sides and fit between the back of the front bench and the front of the back bench. Most people who build this set up are after wide open floors and use the small/long compartments for rod storage.
With a boat like yours, I'd go for option # 2. This does limit you to only 2 fishermen in the boat but the open space is very nice.