What trans cooler are you using? On my 4.7 Dak, I have a Hayden with internal thermal bypass. I tapped into the top line, trans return line coming out of the radiator. In theory, that should act as pre-heater when cold (as radiator heat up, trans fluid heats up) and internal bypass the Hayden cooler. When hot, the radiator should pre-cool and then the external Hayden should cool more. Has worked well for 15 years on my 4.7 Dak, anyone see a reason why it would work on my 5.9?