Cast iron is rigid , stable and doesn't flex much, before breaking. That's why it used in machine tools. If you adhere to the theory of it flexing on a engine stand, imagine what the torque would do to it, in the car.
That said, the old cast in bell housing of the 59a and earlier flat heads, COULD break the bell housing off, when hung by that end, on a stand and did on some of them. Ford's FH mount used the side exhaust ports to mount it on a stand.