Personally I would still suggest going 2" for the amount of weight. If you do use 1-1/2 then be sure it is actually HSS (structural steel, cold forged). If it's just hot rolled square tubing 1-1/2 is going to be pretty sketchy. As others have suggested extra columns is a good idea as well as adding angled supports at all corners to reduce the load on the joints and keep things square.
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