Merger Activity Hits Record High Johnson   Johnson's \$25.4 billion takeover of Guidant Corp. and the \$35 billion union of Sprint Corp. and Nextel Communications Inc. are leading the busiest quarter for mergers in more than four years. &lt;FONT face="verdana,MS Sans Serif,arial,helvetica" size="-2" color="#666666"&gt;&lt;B&gt;-The Washington Post&lt;/b&gt;&lt;/font&gt;