I think I understand your idea, and its exactly what I would do.
I had a Grumman that was laid out just like yours. I only had a week to put some decks in it before a trip, so I just slapped it together .... here is what I did.
If I had it to do over again, that rear floor should have been raised to be a rear deck, and set at seat level. It would have been super easy to do ... a few stringers like in the front and done.
Yours is a bit different because the rear bench has been removed. You will have to fashion a beam going from side to side as well as stringers going front to back. Fortunately though, you get to choose exactly how far back the deck goes .... you'll want to leave some open space back there for access to tanks, batteries and what not.
Honestly, if you leave the rest of the seats in, you won't need that much aluminum to do the bracing for the decks.
A lot depends on if you want storage in your decks with hatches ... storage is really nice, but it complicates the design quite a bit and adds a lot to the price.