I'd use a piece of rectangular mild steel tubing instead of trying to use a sway bar. Couldn't cost $20 for what you need. If you decide to weld on the sway bar be sure the welds are done right. At least for use as a sway bar the welds are critical, and to be avoided if at all possible. Might not be such an issue using a sway bar as a cross member, but still... just saying.
I wouldn't use round stock unless the quality is known.