The pilot reported that he was conducting a third maintenance test flight after a new engine had been installed with the airplane’s co-owner and two other passengers on board. The airplane was full of fuel and no preflight weight and balance calculations were completed. During the takeoff roll, the pilot rotated about 51 kts, which was 20 knots below normal rotation speed, and the airplane became airborne briefly before it settled back on the runway. The pilot then added nose up trim, rotated at 55 kts, and climbed slowly. The pilot continued to climb the airplane when the co-owner heard the stall warning horn. The pilot made a left turn to avoid terrain at the south end of the runway and then setup for a forced landing as the airplane was unable to maintain altitude. During the forced landing, the airplane impacted trees and terrain. The airplane sustained substantial damage to the fuselage and both wings. The pilot reported that the airplane was overweight and that there were no preaccident mechanical malfunctions or failures that would have precluded normal operation. A postaccident weight and balance calculation determined that the airplane was about 359 lbs over the maximum takeoff weight.