The pilot reported that during flight, the engine started missing, then lost 10 inches of manifold pressure. He immediately positioned the mixture to full rich, switched fuel tanks and started the fuel boost pump, but the engine continued to worsen. Shortly after that, the oil pressure went to zero and the engine quit. The pilot received radar vectors through clouds toward the nearest airport, but he was unable to glide that far. Subsequently, the plane crash landed in an open field. A teardown of the engine revealed that the #5 piston had sustained pre-ignition/detonation burn through of the ring belt structure between the lower spark plug and exhaust valve positions. The #5 connecting rod had overheated to bearing destruction. Either the crankpin end of the rod or the release cap had ruptured the case. The fuel pump idle adjustment and aneroid had been set to high pressure (lean) and the fuel control needle valve had been adjusted to high flow. These settings would cause preignition/detonation.