Okay, on the idea of conversion...
First of all, Eagle is right, the open system doesn't out perform the closed system. I feel the only real reason to change from closed to open is personal preference / comfort level. Myself, I am more comfortable with the open system (I feel I understand it better) so about eight months ago I converted (I had massive cooling system failure going on, so had to do most of the parts anyway).
So here's what you have to do, the items in bold are for the conversion, the other items are suggestions of parts to change anyway.
Radiator: For obvious reasons.
Thermostat Housing: The Aux Fan switch on the HO's is on top of the thermostat housing, not the radiator.
Thermostat: Since you've overheated it I would change the thermostat just to be safe.
Heater Control Valve: Again, the design differs between open and closed.
Water Pump: If it's never been changed I would go ahead and eat the cost and change it out, it just makes sense.
Flush The Cooling System: No sense in changing the radiator without getting all the crud out of the engine
and the heater core, do not forget the core!
Fan Clutch: Again, it's about time, you're in there, and it's quite possibly bad.
Auxiliary Fan Control Switch: That mounts on top of the thermostat.
Serpintine Belt: Again, while you're in there you might as well change it.
Idler Pully: Just get your hands dirty once.
Coolant Recovery Bottle: Again, for obvious reasons. Here you can either use a newer XJ one or just pick up a cheap one at AutoZone (or even the bone yard). Personally I went for that "stock look" and bought a newer XJ bottle.
Heater Hoses: These boys also differ. Do yourself a favor and buy the OEM ones from the dealer, they're pre-bent and make the job much easier.
For my conversion I told all parts guys that I had a 1995 XJ.
Now, on control of the aux fan, you should just be able to use the new switch on teh thermostat housing, you will have to splice in longer wires to the aux fan switch and a different style connector (it's common on GM cars).
As you can see, it's an involved process to do the conversion. If I were you I would just change that clutch out and see what happens. I'd also get a new bottle (it's cheap, so it won't hurt too much if you end up changing over to open down the road) and put it in. If you end up deciding that you have to change the radiator I'd revisit this topic and decide if you want to go though the time and expense of the change. You will have to do some very minor fabrication work to do this. The top radiator brackets between closed and open will not work, but it's easily fixed with just a little bit of metal 1" stock, cut it to length, and drill two holes. Also, you will have to come up with a way to hold the new coolant recovery bottle down.
Sequoia