The private certificated pilot was landing a tailwheel-equipped airplane on a gravel bar. The pilot reported that his airplane is equipped with 30-inch tundra tires. He recently installed extended landing gear to add distance between the bottom arc of the propeller and the ground. He also added new brake cylinders equipped with a 'booster' cylinder. During the landing roll, the pilot said he applied the brakes too hard, and the airplane nosed over. The airplane received damage to the rudder, and the right wing lift strut.