On August 13, 1999, at 1311 eastern daylight time, a Cessna 208, N193GE, was destroyed during a precautionary landing near Hillsborough, New Hampshire. The certificated airline transport pilot received minor injuries. Visual meteorological conditions prevailed for the ferry flight which departed from Manchester, New Hampshire (MHT), and was destined for Denver, Colorado (DEN). The flight was operated on an instrument flight rules flight plan under 14 CFR Part 91. The pilot reported he flew the airplane to Bangor, Maine, on August 11, 1999, for the installation of two auxiliary fuel tanks. The installation took place on August 12 and 13, and the pilot flew the airplane to Manchester, Hampshire (MHT) where the auxiliary fuel tanks were filled. The Federal Aviation Administration (FAA), Portland Flight Standards District Office had issued a special flight permit for the overweight operation of the airplane. At MHT, the airplane was serviced with 654 gallons of Jet-A. The wing tanks were filled, and the auxiliary fuel tanks (205 gallons each) were serviced with 190 gallons each, which brought them to within 1 inch of the top. The pilot reported that the gross weight of the airplane was about 10,000 pounds. The takeoff and initial climb were without incident. As the airplane passed through about 800 to 900 feet, the pilot smelled turbine fuel. He stopped the climb and attempted to locate the source of the fumes. From his seat in the cockpit, he could not identify the source of the fumes. He requested radar vectors to the closest airport and initiated a descent. He then noticed about 1 inch of standing fuel on the floor of the cabin, and turned off the radios. During the descent, the amount of fuel in the cockpit area continued to increase, so the pilot elected to land in an open field rather than continue toward the airport. He reported that due to fuel fumes and raw fuel in the cockpit, his vision was blurred, his eyes burned, and he had difficulty breathing. He positioned the airplane for landing into the wind. On final approach, the fuel was up over his ankles and his sectional charts were floating on top of the fuel. After touchdown, he applied maximum reverse thrust and brakes. During the ground roll, at a speed of about 40 knots, he observed a ditch ahead of him. He had no further memory of events until he was being treated by paramedics. A nearby airplane observed the landing and reported that the airplane nosed over during the landing roll. After about 5 minutes, the pilot exited the airplane and crawled away. About 5 minutes later, the airplane started to burn. Medical personnel arrived about 15 minutes after the accident. An inspector from the FAA reported that the airplane traveled about 400 feet on the ground prior to encountering a ditch which measured about 60 feet wide and 3 feet deep. The airplane crossed the ditch, and continued for about another 120 feet and came to rest. The nose landing gear had separated from the airplane. The airplane was consumed by fire except for the outboard wing panels and empennage. Documents from the FAA and interviews with the pilot revealed the two tanks were mounted in the cabin laterally, and secured to the floor with tie-down straps. The takeoffs for the fuel lines were on the back side of the fuel tanks and not visible to the pilot. The two tanks fed into a common manifold, which was used to replenish the right wing fuel tank. The engine could not be fed directly from the auxiliary tanks. The pilot reported that he had not actuated the auxiliary tank system on the flight. Examination of the supporting documents for the tank installation, furnished by the facility that performed the work, and interviews with the pilot, revealed the following; the fuel tanks were installed laterally; however, the FAA Form 337 showed the fuel tanks installed longitudinally; the instructions issued to the pilot for the use of the auxiliary tank system did not cover all contingencies, or limitations; the pilot reported that the 3 auxiliary fuel pumps used to transfer fuel from the auxiliary tanks to the right wing tank were secured to a platform. However, the platform was not secured to the airplane. These items were discussed with the FAA Principle Maintenance Inspector (PMI) who had oversight authority for the facility that performed the auxiliary tank installation. The PMI reported that the facility has subsequently changed their procedures.