The discharge line to the the cooler is the forward most union on the drivers side. You will find the return union +/- 50mm behind it. Note the front union is hard to access on an auto box as the shift linkage sits snug up next to it.
I went to an auto specialist who hooked up his computer to my D40, there are 2 senders as standard in the auto boxes, both were reporting around 67-70 degrees C after a normal run.
I have since had a temp gauge fitted with the sender coming of the return union from the cooler, I know this is not ideal but the front union is not accessible. The auto box shop advised that if there was an overheating issue the sender on the return union would still indicate a significant increase in normal temps. Normal on my ute seems to be around 70 degrees.
While it was on the stand they checked out the auto trans cooler and reported that it was actually a pretty decent size, so no real point changing it unless you want to remove the initial radiator circuit and go competition driving.
