I'd second what Dan said. Sandwiched areas are places where rust loves to hang out and grow. Even if you sandblasted all the rust out, moisture will get in there and rust will start to grow again.
The floor should be welded to everything around it, as should the cowl panels. They all contribute to the structural strength. Think of a cardboard box with the flaps just folded over and not taped. It'll flex big time, and you don't want your cab doing that. Fabricating your floor out of 16 gauge like you did is fine - a little overkill - but it does need to be welded in.
