Not sure what the design goal here is, or the cars intended use. I would Not "roll Over" the 2x3. I would not use 1/8"/11g tubing. 2x2 would solve a lot of issues I suspect. Nobody here in over a decade can clearly delineate what forces the SFC's are exactly resisting, so the thinking "it can't hurt" likely applies. I would weld the 11G? end plates onto the TB crossmember first, and also add a center hole to weld/hold down the center of the plate, and then weld /fit the SFC to that, and weld in place. You may have a tough time welding the top fully, but anything you get on that surface would help.

Here is one of my cars with 2x2, 3 pics starting at pic #8:

https://board.moparts.org/ubbthread...e-62-polara-500-cruiser.html#Post1554319