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Todd,
Frankly I would be very suprised if the self-drilling self-tapping screws like what I showed in the photo would pop (come loose) over time. I could believe that they might for the thiner metal of the original cross members but not for the thicker metal that you are using for your tubing. Again predrill the pilot holes and I think you should be just fine using elevator bolts where you have access and the screws where you dont. Some of my old floor was held down with screws and I had a hard time getting them out.
Putting angle brackets on the frame would tend to be in the wrong places too since the plywood joints would fall on the middle of the cross-members. If you really need to have elevator bolts then you could perhaps drill access holes in the tubing that are large enough to slip in the head of an air drive socket wrench to tighten the nut on the bottom end of an elevator bolt. In my opinion welding on extra stuff is just not necessary. Also I happen to think that the most important attachment points are the ones around the edges of the floor where the body is directly above. The outrigger attachment points for sure can be elevator bolts.
Malcolm
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