I have done this swap twice.
The mechanical fan has a thermo clutch that is activated by heat coming off of the rad, it will not be effected by your rad swap.
However the electrical fan on the closed systems is controlled by a sensor in the rad and the open system is controlled by a sensor in the t-stat housing.
The first conversion I did I was able to order a rad that had a filler neck but still had the bung in it for the sensor. I ordered it from Parts Source and oredered it for a '91 XJ WITHfiller neck, there were two option in the computer for that year, with and without filler neck so I'm guessing that is was a change over year.
I have not been able to get this rad again so the second one I did I picked up a newer t-stat housing with sensor(and wiring!) from a wrecker and extended the wiring from were it was in the rad to reach upto the t-stat housing. This is usually the easist option. While you are at it you may want to wire in a switch is turn it on manually, the sensor used in the t-stat is not very reliable and has failed on me a few times.