A little word of advise.

The 2x3 " square tubing is suppose to be inserted in the subframes along the 2' width. I use 2x2 inch square tubing and inserted them inside subframes. The 2x3 " tubing will protrude through the floor in back. While the 2x2 will not. If I understand you right. The 2x3 " tubing is not laid flat.