Mrs. Dulles Fontana, formerly Miss Margaret Dulles, who once sued her husband to prevent him from collecting $75 a week support from her, died today in the Barclay Hotel after a long illness. She was 56 years old, daughter of the late Mr. and Mrs. Andrew Cheves Dulles, member of a family well known in Philadelphia since Colonial times. A brother, William Crothers Dulles, went down with the Titanic. She was married in 1908 to Ettore Romano Fontana. A year ago she started divorce proceedings and later brought suit to nullify a contract under which she had agreed to pay her husband to stay away from her. He died in Atlantic City in September.