Your ex and the trans are heating the floor which does not mean that the trans is running too hot. Putting in a T of the correct type to add a sensor in the output cooler line(the front one) wil give you the hottest reading but the pan works good as said you get a baseline that you're comfortable with & watch for any spikes. I'd use the largest cooler you can fit plumbed after the rad(or bypassing it, my preference) and good airflow thru it.