I used 2-2"X2"'s on 6" centers down the middle to catch the bolts for the front seat base, then another 2"X2" on each side of the hull, but mine wasn't quite as wide as yours. I still used 3/4" decking. It's not that much more in cost & weight, especially after factoring additional supports.
lol, trust me, you don't need to be worried about weight in that boat :wink:
ST