Ford Motor Co. said it would use a new diesel engine from Navistar International Corp. in its large trucks, as competition for buyers of heavy-duty vehicles stiffens. Ford said the 6-liter Power Stroke V-8 would offer more power, better fuel economy and lower emissions than the 7.3-liter Navistar engine it will replace. The engine, rated at 325 horsepower and 550 foot-pounds of torque, will be offered in F-Series Super Duty pickups and the Ford Excursion sport/utility starting early next year. Ford sells about 250,000 of the heavy-duty diesel engines a year, mostly to buyers who tow heavy loads.