From what I can see in the picture, no problem going with a longer frame on your 16' boat. I also see your oar towers mounted quite far back from center of the raft. Is this the reason you want to reconfigure the seating arrangement? I'd remove the thorts when hauling gear.
FWIW....I'd add at least one more D ring to each side of the boat for frame attachment. Maybe two on each side if I'm planning to really load the boat up for say a Deso trip or any multi day excursions. Four corner straps to a frame loaded with gear says to me............ the frame might not follow the boat back to the upright position when trying to reflip it.