Abstract: A kit and a method are provided for modifying an automatic transmission such as a turbo-400 hydromatic transmission. Before modification the automatic transmission couples an engine crankshaft 4 to a forward clutch drum assembly 3 through a fluid coupling such as a torque converter. This kit provides a lockup shaft 10 selectively coupleable between the engine crankshaft 4 and the forward clutch drum assembly 3. As modified, the forward clutch drum assembly 3 can be selectively driven either off of the torque converter turbine 32 or off of the lockup shaft 10 coupled mechanically to the engine crankshaft 4. The lockup shaft 10 extends through intervening structures from the crankshaft 4 out to the modified forward clutch drum assembly 3 where it connects to a lockup hub 20. A drive tube 30 of hollow cylindrical construction surrounds the lockup shaft 10 and is secured to the torque converter turbine 32 within the torque converter housing 8 which is driven by the torque converter pump 9.