The pilot loaded the 2 passengers and gear in the airplane to the extent that it took successive attempts to close the door due to the gear which 'filled up the airplane to the roof.' The rear passenger had to lay on top of meat and camp supplies with 2 dogs in his lap. The airplane was estimated to be 772 lbs over max gross weight at takeoff. After takeoff, the airplane climbed slowly for approximately 55 minutes at takeoff power. A 'sledgehammer' sound preceded the appearance of oil flowing into the cabin area. No reduction of power was observed. The oil pressure began dropping rapidly, and the pilot made a precautionary landing on a beach. The airplane nosed over in grass. Engine teardown examination revealed that the #6 cylinder had failed through the head-to-barrel threaded joint. Metallurgical examination revealed that the failure was the result of fatigue cracking in the cylinder head casting. This type of failure occurs when the cylinder is operated at elevated temperatures (above maximum allowable) for a considerable period of time.