A spokesman for New York-based Morgan Stanley & Co. declined to comment on the sharp rise in the company's stock in heavy New York Stock Exchange trading. The stock rose $6.125 to close at $76.875, with about 343,000 shares being traded. Market sources said there are unspecific rumors that the company may be a takeover target. In a related development, Shearson Lehman Bros., a unit of American Express, may make an attempt to take over, E. F. Hutton, one of its biggest Wall Street rivals, according to stock traders. Thomas Brown, an analyst at Smith Barney, Harris Upham & Co., said he viewed a combination of American Express and Hutton as a "distinct possibility."