Complex sentence: A customization firm in the United States based their entire business on converted normal 3000GTs into soft-top convertibles, these machines should not be confused with the Spyder, which was Mitsubishi designed and built.
Split into simpler sentences: A customization firm in the United States based their entire business on converting normal 3000GTs into soft-top convertibles. These machines should not be confused with the Spyder, which was Mitsubishi designed and built.