The converter in the MT643 only locks in 3rd and 4th gears. So, if you're climbing steep hills, it's probably still a good idea to add a transmission cooler.
I have one on my bus, because we are driving through the mountains of Colorado with an AT542. It was required. We mounted it in the front bumper, only had to add one short section of hose from the radiator intercooler over to the oil cooler, and reused the old return line. Doing this while swapping the transmission would probably only add 10% more work.