Combine the sugar, evaporated milk, butter, and salt in a 4-quart saucepan. Stir well. Bring to a full rolling boil (that cannot be stirred down) and boil 4 minutes. Have the chocolate and marshmallows ready in a mixing bowl. If you prefer, cut the marshmallows in half with greased scissors, or use tiny ones. Pour the sugar mixture over the marshmallows and chocolate, and stir until they are melted and well blended. Stir in the nuts, and turn into a well-buttered pan about 10 × 14 × 2 inches. Cool at room temperature. This will take about 8 hours to set firmly. Cut into serving-size pieces after cooling.