"That top nut" What does that mean? You have the two banjo fittings on the transmission that are the two transmission lines running to the radiator and on the right side of the car aft of one of the banjo fittings you have the base of the dipstick tube. The dipstick tube is attached to the case with a 10mm bolt.If you lossen the bolt the tube will raise out and I'm thinking there is an o-ring on the base of the tube.
Is something else leaking?

PS. In the very first picture in this thread looking at the bottom of the radiator there are two plastic fingers sticking up. Those fingers sit in the slot I spoke of earlier in the frame of the car. If you have those fingers in the correct slot then the radiator is in it's proper place.