This is not advice, only some limited experience.
I built a replica Cobra and was trying various sway bars. I would visit the junkyard and buy something close to fitting, then cut and weld to better fit. As long as the welding stick is stronger than the spring material, the weld will stay stiff while the bar does the twisting. In my case, I only cut/welded the arms, never the center section. In fact, I often added a piece of flat stock that allowed me to drill several holes for the end link to attach to. Again, not advice, but if the arm is stiff (by using the proper welding rod and/or adding braces) then it won't bend, but the center section remains the torsional spring you desire. How well the weld area is prepared (chamfered ends, etc) makes a difference.
Alll in all, unpleasant surprise!