You want to know the temp of the fluid in the pan, not the line. The transmission gets the fluid from the pan and runs it everywhere before it sends it to the cooler. The cooler is just an auxillary add-on feature. The pressure of the oil in the cooler does no work and is a low pressure operation. Worry only about what the temp of the oil is in the pan. Personally, I would never remove the water cooler from the radiator because the heat exchange is much faster than the air exchange of the heat, that's why these big engines are not air cooled. If the air cooler was mounted before the water cooler for the transmission fluid you will not get any benefits of the additional cooler. The fluid should flow through the radiator and then to the supplemental cooler and then back to the transmission.