The pilot lost control of the airplane when the steering arm from the tailwheel assembly failed during the initial takeoff roll. The 20,000-hours airline transport rated pilot reported that about 250 feet along the takeoff roll, when the airplane was accelerating through 40 to 45 mph, the airplane started to pull to the right and his full left rudder input was unable to maintain directional control. The 2,600-feet long by 80-foot wide grass airstrip featured a 4 to 5 foot deep ditch along the right side of the airstrip. The pilot added that he attempted to get airborne prematurely and the airplane was able to clear the ditch; however, after clearing the ditch, the right wing became entangled in some tall grass and the airplane subsequently impacted the ground. The vintage airplane came to rest in an upright position and the pilot and passenger exited the airplane unassisted. An examination of the airplane revealed that the fuselage sustained structural damage. A detailed examination of the tailwheel assembly confirmed that the steering arm was completely fractured. The wind at the time of the accident was reported as calm. The pilot had accumulated approximately 30 hours in the same make and model.