From my experience in welding up cracked heat treated steel (some, but not very much), a second crack will develop on the edge of the weld where the strength has been lost from the heat of the weld. I'm sure there are ways to get around this such as heating the steel up with an oxy, then welding it then quenching it or something.
Since you've swapped to another one anyway, why dont you experiment with the cracked one by welding some angle or RHS or Box section to the bar to give it some added torsional strength, hopefully more than the spring steel added initially.
If you're just going to chuck it out, send it to me and I'll do it. My torque is distributed more evenly through the revs rather than a big slap in the face like yours would.