Well got it done, I'm happy so I guess that's all that matters!
I shortened it up to the first rib as I intended...it just took up to much floor space otherwise. Ended up doing the edge support by riviting angle to the side of the gunwale - it supports the ply on knife edge, not ideal but more than adequate in this case with the small spans involved.
rigging the vertical posts to the existing ply floor was more difficult, but I like my solution even if it's a little ways from perfect. Hard to explain, hopefully you can see it in the picture. I cut a small piece of angle and attached it to the floor oriented fore-aft. I simply riveted my vertical piece of angle to the piece on the floor with a single rivet so they could pivot to the proper angle. Once achieved I added a few more rivets to lock it down. It's plenty solid.
Decked it, added the vertical piece and carpeted all. Looks great, is the perfect function (life vests and anchor are all in there), and it's plenty strong and sturdy enough for my 200+lbs to take a couple foot step down off a river bank onto it with no problems.
Thanks for the tips, it all helped!