Kenny Brown was a decent place for these items. Just recently it was known through the news that "Kenny" himself became ill and some people at his business tried to steal his business. It's been a problem for his company. He used to sell "matrix braces" and "jacking rails" to complete the kit. There are better but KB are up there for chassis stiffening.
He went beyond the standard subframes to his supersubframes along with the kit...
Here is a stock photo of the entire kit on another car which may not be a Mustang. His metal is much more strong than the metals used in this picture of some application. My kit to follow...
My kit before install...