The pilot reported that prior to departing on the test flight he checked all of the rigging, and the oil and coolant levels. After performing a takeoff and landing, he stopped and rechecked all fluids. Then he departed to a test area to evaluate slow flight and turn characteristics. The pilot then turned back towards the airport. The pilot stated that it was at about this time that he observed the coolant temperatures rising, and within a few seconds he could smell coolant. He reduced power, set up an approach to a level field and monitored the temperatures to avoid overheating the engine. The pilot reported that he landed in a bean field, and about 100 feet into the landing roll, the bean plants grabbed the landing gear; the airplane subsequently nosed over onto its back. The pilot reported damage to the empennage and fuselage, and that there was no post crash fire. The pilot further reported that the cause of the engine overheating was the result of a coolant leak from the left intake manifold gasket.