The pilot initiated the takeoff roll, and as the airplane accelerated, the pilot's seat slick back to its aft most limit. As a result, the airplane departed the runway, impacted bushes, and incurred substantial damage. A review of the airplane's maintenance logbooks revealed that a new set of seat rails had been installed in the airplane about 31 flight hours prior to the accident. Examination of the seat rails revealed that the inboard rail had been modified with the addition of 5 locking pin holes. The airplane was originally equipped with a single locking pin type seat; however, a dual locking pin type seat was installed. Selection of any seat position, other that the three most aft positions, resulted in partial engagement of a single locking pin, while the other locking pin rested on top of the opposite seat rail. This combination of seat and seat rails was improper.