Economic Index Up In October
Commerce Dept repts gain of five-tenths of 1% in its composite index of leading indicators, signaling prospects for continued growth in econ; advance is 3d consecutive monthly increase; dept's chief economist Courtenay Slater comments; sampling of opinions of private economists who see slump ahead; Fed Reserve chmn Miller says productivity should be helped by Pres Carter's plans to reduce Fed spending to 20% of GNP by '83, speech to Amer Council of Life Ins; says some $65 billion to $70 billion will thus be made available for investment by private sector over next 5 yrs; graph on Index of Leading Econ Indicators (M)