By John S. McClenahen Like many highway repaving projects, corporate balance sheets are likely to be sporting "Under Repair" signs a bit longer than many manufacturing executives or economists expected. First quarter 2003 "flow-of-funds" data from the Federal Reserve, published last week, "actually showed a mild deterioration in private-sector balance-sheet quality, despite all the talk about how finances are actually improving," points out David A. Rosenberg, chief North American economist at Merrill Lynch & Co., New York. He notes that debt relative to net worth or capital rose to 53.2% in the first quarter of this year. "Only seven other times in the past 50 years [has] this ratio [been] as high as it is today," says Rosenberg. His bottom line: The balance-sheet repair process will retard GDP growth and "probably take longer" than expected.