Well Shelby built his own car before the Mustang existed. Without the Mustang he for sure would have been involved either with his own chassis or something elseChicken/Egg.
Would there even have been a Shelby if not for the Mustang?
Sure there was that Olds thingy, and the FWD Dodge whatever.
But without the Mustang would Shelby have even been involved in automobile manufacturing?
I think not.
We will never know.