I would replace the radiator with the one that was designed for the deisel engine. The engineers used a different radiator for a reason. Also, replace the thermostat, sounds like it was stuck open or the wrong temperature. All three of those conditions could cause overcooling. You may have to pay more to order the correct thermostat at a dealer, but at least it will be the correct one. I know some parts stores sell aftermarket thermostats that are 10-15 degrees cooler than a factory one. That's what I'd do if it was my car.