Mike;
The material is not too important, as long as it is steel. Chrome-moly, mild steel, etc doesn't change the stiffness for a given size. Depending on how far the bar is twisted will determine the stress and therefore the strength required. I'd suggest simply using mild steel tubing as it is lighter than a solid round bar of equal stiffness. The torsional stiffness is mostly determined by the material near the outer circumference of a round shape so a solid bar contributes little or no stiffness near its center. Tubing, on the other hand, has its material distributed at its outer edges already-- an ideal stiffness-to-weight ratio.
Regards, Neil Tucson, AZ