President Clinton has approved final pollution-control rules that will drastically reduce heavy-duty truck and bus emissions over the next 10 years. Under the new rules, the big diesel-powered vehicles, which are the heaviest polluters, would be required to reduce their emissions of soot and smog-producing material by more than 90 percent. The new regulations are tougher than the standards set for cars and light trucks a year ago. The changes are to take effect in 2006, but they will be announced by the White House and the Environmental Protection Agency on Thursday, Dec. 28, as part of final regulatory action designed to withstand any challenge by the incoming Bush administration.