You don't weld ANYTHING to the torsion bar. Just the plate on the bottom of the LCA.
And yes, you can use any kind of metal, you can make your own plates. Really you don't need a plate that runs the length of the LCA, just a few straps or bars that connect the open bottom of the LCA. You can't weld a solid plate on though, because you need access to the torsion bar adjusters.
The video covers everything, and explains the process very well.
