After landing the Captain was taxiing the airplane to the parking area to off load his cargo of fuel. While the Captain was maneuvering the airplane in the parking area the left wingtip struck a vertical steel building support. The Captain attempted to back up the airplane through the use of reverse thrust. While backing up, the left elevator, stabilizer, and vertical fin struck a steel fuel tank being stored near the parking area. The Captain stated that the parking area is tight for a DC-6 airplane.