The 722.6 is used in the chryslers. Same transmission is still used in the twin turbo v12 cars for it's power handling capabilities. I run them in my cars.
These can be built to handle in excess of 800hp. Stock transmissions can handle up to 500hp and up to 700hp in the v12s.
Many different variants depending on what the application was but almost all of them can be built up pretty strong.