Jim I noticed you said you were going to have hand rails. Why not make them from the same size tubing you are already using and make a truss out of them. This will not totally stiffen the structure as many have pointed out, but perhaps will be enough to live with. I recommend you use your backhoe to lift the end up and hold it while you fab the hand rail trusses. What ever distance it sags naturally down lift it that same amount uppered above level this will preload the structure and take out the sagging and the spring board effect.
