Even as Greece desperately tries to avoid defaulting on its debt, American companies are preparing for what was once unthinkable: that Greece could soon be forced to leave the euro zone. Bank of America Merrill Lynch has looked into filling trucks with cash and sending them over the Greek border so clients can continue to pay local employees and suppliers in the event money is unavailable. Ford has configured its computer systems so they will be able to immediately handle a new Greek currency. No one knows just how broad the shock waves from a Greek exit would be, but...