Even corner braces would help...
I agree more cross braces across the top, even though the tank will probably be on the outside edge of the steel, it's seems like it would be worth the extra effort! The tank will probably right around 100+ pounds plus close to 1500 pounds of water and another 3-400 in live rock and substrate. So my best guess would be it would have to hold atleast 2,000 lbs... Not sure exaxtly how you would test the stand, but think of it as the stand should be able to hold a small car and you should be good. Good luck and cant wait to see it all set-up!