N3025W departed on a ferry flight with an unsecured, temporarily installed fuselage fuel tank. A computed C.G. at takeoff showed the aircraft about .9 inches aft of the aft limit. The aircraft had climbed to 7000 feet when the controller saw the aircraft making erratic turns. The pilot said 'We have problems.' There were no further communications with N3025W. The aircraft subsequently broke-up in flight. The forecast the pilot received included moderate turbulence and rainshowers. The ferry tank installation had not been approved. The pilot was new to this type of work and reportedly did not know how to work the ferry tank system. The takeoff position of the ferry tank was estimated. It had been pushing out the fuselage baggage door and had to be pushed back in after being filled. It was considered likely that the unsecured tank would have migrated, changing the C.G. in flight, altering longitudinal control forces, and causing pitch control problems in turbulence.