There is a guy in UK that will supply valves to your spec. He uses whatever is close and machines them to suite. Surley there is someone out your way doing similar? Try classic car forums, they are the likely market.
Its a pretty safe bet that Ford used the valves in another more common engine as well as yours. A search in a Ford agents or engine parts supplier may turn it up.
Making a larger valve smaller at the head, reducing the stem diameter if needed and shortening/machining the collet end is an old and well established practice.