I am with RobertF on this one. The steel that broke appears to be about 3/16" thick. It takes a LOT of force to shear steel like that ... it takes alot to just bend 3/16" flat bar. I realize how much force in on a trackbar.
Now, a poor weld could have weakened the steel, but had that happened, the steel would be visibly different ... both composition and color. i have no personal experience with Rusty's stuff, but if i were to build my own mount, that design with 3/16" flat bar is exactly where I'd start ... although, since i tend to overdo everything, I'd use 1/4".