What is the largest 6-car-sets for fiscal year 1968?
{'header': ['Fiscal year', '2-car sets', '4-car sets', '6-car sets', '8-car sets', 'Total vehicles'], 'page_title': 'Tobu 8000 series', 'page_id': '', 'types': ['real', 'real', 'real', 'real', 'real', 'real'], 'id': '1-19255192-1', 'section_title': 'Production', 'caption': 'Production', 'rows': [['1963', '0', '15', '0', '0', '60'], ['1964', '8', '15', '0', '0', '76'], ['1965', '21', '21', '0', '0', '126'], ['1966', '27', '27', '0', '0', '162'], ['1967', '39', '36', '0', '0', '222'], ['1968', '47', '43', '0', '0', '266'], ['1969', '58', '51', '0', '0', '320'], ['1970', '62', '55', '0', '0', '344'], ['1971', '62', '42', '13', '0', '370'], ['1972', '62', '41', '17', '0', '390'], ['1973', '63', '41', '22', '0', '422'], ['1974', '68', '41', '25', '0', '450'], ['1976', '70', '41', '31', '0', '490'], ['1977', '70', '41', '31', '5', '530'], ['1978', '70', '42', '31', '9', '566'], ['1979', '70', '42', '31', '14', '606'], ['1980', '73', '45', '32', '16', '646'], ['1981', '75', '47', '33', '16', '664']], 'name': 'table_19255192_1'}