There shouldn't be an issue using square tube. I first considered using square tube but decided to use the flat stock so it sat below flush with the frame. I found a manufactured trike that had all round tube and similar front supports and I didn't like how it looked as much, so I played around with the design and liked the curved look more than the straight line. I thought about bending a radius into the square tube but couldn't figure out how, I also considered getting the flat stock waterjet cut, with some circle or oval pattern to cut some weight, keep the strength and look cool, but I couldn't find a place and wanted to get the trike finished.
Popshot is correct, the pulley will need to be moved. I had to move mine forward to avoid hitting the support. Because you're moving it forward it will affect the height of the chain,, I added a small spacer between the pulley and frame to drop the pulley a little lower so it still cleared everything, you could also use a larger pulley. I would advise you to only tack in place as possible until you know the chain travel and everything will clear and is correct before welding it.