The pilot of the Boeing 75 reported that he taxied south on the taxiway toward the runway, using S-turns to clear for traffic, and followed about 200 ft behind a Piper airplane. He also noted that there was a light twin facing north conducting an engine run ahead of the Piper. The Piper pilot made a 180 degree turn on the taxiway and taxied north, which the Boeing pilot did not see. The pilot of the Piper reported he conducted an engine run-up along the west edge of the taxiway near the approach end of the runway, facing into the wind (north), as there was no defined run-up area at the end of the runway. The right wings of both airplanes collided which resulted in substantial damage to both wings. It is likely that the Boeing 75 pilot did not anticipate that the Piper in front of him would turn and stop to conduct a run-up so far north on the taxiway which led to the collision between the two airplanes.