i have the exact same boat as you only older. the best way i found to build the deck with stability in mind is to brace it so that the deck lays on top of the middle seat and then fits right under the lip of the front seat. this gives you a level deck and allows adeqaute storage underneath. With the deck at this height it is stable enought to walk and cast around the deck by yourself with some one else fishing of the back seat. check out my post " florida jon boat " in this forum to see what i mean. i felt like it was much more stable with a deck than without it, i think standing on the ribs and bow incline throws off the balance.
https://tinboats.net/forum/viewtopic.php?t=2233