Most US vehicles have the automatic trans fluid running through the radiator for cooling.
I would buy a new radiator and get the line hooked back up to it.
I would have your transmission serviced every 50,000 miles. maybe 30,000 with lots of towing?
I hear of lots of GM vehicles loosing their auto trans before 100,000 miles. After a good rebuild at a good trans shop they seem to run much longer?
Many large Ford, GM and Dodge trucks are all coming with beefier transmissions now.