During cruise at about 1000 feet agl the engine began to run roughly and the pilot observed falling oil pressure indications. A severe engine vibration then developed and the pilot, after selecting a suitable forced landing area, shut down the engine. He delayed lowering the landing gear until he was satisfied that the selected area had a firm surface. The gear was selected down on very late final, but only the nose gear had time to become partially extended before touchdown. Lack of sufficient tension of a nut securing a conrod bolt of Number 1 cylinder allowed the conrod bearing cap to loosen causing loss of oil pressure and subsequent engine failure.