Gussetts on the outside of the tubing is reccommended. We have used this material in the past ans under heavy loads (collisions) the webbing has a tendency to break out where the nuts are compressed against the webbing.
We ended up with gussetts about 4" on a side with two connectors in each side of the gussett. THis beefed up the corners enought so we no longer had structural problems.
Just a thought...