Initial reinforcements for Main Hoop.

The reinforcement idea is a good one, but you'll want to ensure the tubes are located flat on the steel, the pictures suggest you will be trying to match up angles.  It can be done, but is tougher to get a good fit.  Your design looks good.  Building corners like that is a great way to increase strength in iffy areas.  I wouldn't introduce 1x3 stock.  Open boxes collapse under load, so you would have to close the ends, which is a pain.  All the 1x3s will do is transfer the load down to the thin stuff anyhow.  I would just build the corners and weld in the hoop.

The welds in those pictures are a bit more concerning.  Lots of inconsistent beads and poor penetration.  I'd expect to see some of that when welding thick to thin, but it looks like some of those welds are thick to thick.  You'll have a tough time in tech with welds that look like that, especially if they are on the tube joints.  You can sometimes get by with rough looking welds on the spreader plates, but it sucks betting your weekend on it.