No iron: Galvanic corrosion caused by different metals touching and getting wet. Aluminum angle is likely available where ever you could buy angle iron.
Closed end blind or solid rivets OR SS bolts and nylock nuts work, depends on location, if you're prepared to buck solid rivets and whether you mind the bolt and nut sticking up off the hull @1/2"+/- ,rivets are essentially flush.
Use a sealant like 3M 5200 below the waterline for rivets or bolts, 3M 4200 above. Dip the rivet/bolt into 5200, push it home and crimp/buck it or install the nut. Messy affair, but helps prevent future leaks.
You wont be the only one EVER to use this boat, so regardless of how shallow the water where you boat, or how well you swim, others may not be so lucky. Its flotation foam to keep the hull from sinking, yes, not to allow it to float for normal use. There needs to be enough foam, placed correctly to allow the boat to float level and slightly above the water when fully swamped. Dont skip the flotation foam. You paid for the boat and are working to redo it, you also dont want it to be a complete loss if ever it gets swamped, however unlikely you think that might be. They're called accidents for a reason.
If you are putting a deck across where the top of the bench seats used to be, you are raising the center of gravity. Else I dont see how you'll hide the fuel and gain storage.
I wouldn't remove anything else that you havent already. And dont forget to put back some of the structure to replace the removed benches.
What do you plan to make the deck out of? Dont use pressure treated plywood or PT lumber in your rebuild either.
Seal the deck on both sides and all edges before installing it.
Spend some time reading thru some tin boat rehabs. Even a topic
rebuild of a 26ft tin boat will have useful info that translates to work you have ahead. Structure, fasteners, foam, plywood, sealing and paint...