After landing, the pilot was cleared to taxi his tail-wheel equipped airplane to the ramp and was aware that another airplane would be in front of him as he taxied. As he crossed over an active runway onto the taxiway, he steered straight ahead and did not perform any s-turns to maintain visual contact of the airplane ahead of him. As he cleared the runway, he looked down to change his radio frequency and suddenly realized that he needed to look out for the airplane in front of him. When he looked up, he saw the right wing tip of the other airplane and did not have time to stop before he collided into its tail. The collision resulted in substantial damage to the lower right wing spar. The other airplane sustained substantial damage to the entire tail section including the vertical stabilizer, the rudder, the horizontal stabilizer, and the empennage.