16' Steel Jon Boat

Unless you have the 1/8th steel already I think you'll find that a bit heavy. 1/8th is ~5lb/ft2. But if weight doesn't matter just factor the hull weight into your total displacement. Using the swag method I'd say your hull will easily weigh the same as your total 800lb load if you build it out of 1/8th (add some framing, gunwales etc and weight adds up fast).

Then the experience I found building my little steel skiff (used 16ga), large flat areas are hard to keep stiff. If you don't build a jon boat, but go for a few chines and not flat bottom (even a slight v is massive dif in stiffness). You can get away with thinner ga material.

You could also do what I'm planning, build the flat bottom out of steel, then attach a wood framed floor (I'm just going to use 5/4X4 patio decking) to it for stiffening and damping the big drum (lol its noisy).

PLus 16ga is not strong enough to hold someone standing on it with out deforming so wood floor helps there as well.

Jon boat sure is the easiest thing to build though.