In my mind, 600 HP and 121 MPH does not make sense. It`s been many years since I raced a car with an automatic, but I know many people who found major improvements with different torque convertors. A racer that I know switched from a TCI convertor, that TCI built, supposedly after he told them his combination, to an ATI. Not only did the new convertor give him 6 MPH, but despite going much faster, it did so at lower RPM (less slippage), so he installed steeper gears, and picked up even more performance. What sort of finish line RPM are you turning at 121 MPH?