ByJohn S. McClenahen Japan's economy will likely continue to grow, although not at the 4.5% annual rate of the last 18 months, says the Conference Board. Japan's leading economic indicators declined 0.3% in September, the most recent month for which data are available, says the New York-based business research group. The leading economic index for Japan now stands at 98.8 (1990=100). The leading index declined 0.4% in August and rose 0.2% in July.