OK, I contacted CJ Pony Parts and asked if the Mustang pedal extender might fit the FiST.
I was told that it defiantly will NOT fit the FiST.
Additionally the Mustang extender is only 1.5 inches.
I still plan to build one and I more than likely I will use a similar design.
However, I might just order a Mustang part to see what the differences are.
It may be easer to modify the Mustang part rather than make a new one.
See this thread.
https://www.fiestastforum.com/threads/clutch-pedal-extender-install.22930/