Hi,
Riverstone radiators in Sydney made my 3 row copper/brass custom one for about $700.
They got the core from Qld and added some custom tanks.
I think some guys got ones made cheaper elsewhere.
I just gave them my crappy one with the plastic tanks and they copied it.
Make sure they fit the air vent bleeder pipes, he forgot first time round for me.
Extra core row adds 50% more water in the radiator so, for same engine speed, water flow rate in engine is same, but with larger cross-section in rad, water spends 50% more time exposed to air flow before returning to the engine. A little more water iin there too.
Be absolutely certain your electric fan is working, should come on a little while after the A/C is on. Some diesels misssed out on thelectric fan, if so, fit one!
Be certain the clutch fan works too.
With the diesel running at low revs the water pump drag is the least of your problems. All XJs are mainly limited by the amount of air flow rather than anything else.
Mine has no problem with water flow. When the engine is hot the rad is hot too. The air flow is the limiting factor by a long shot.
Assuming you block/head are real clean inside coolant should not be an issue.
To prevent head damage be a demon on finding pressure leaks, if your cooling system doesn't hold pressure then your boiling point is much, much lower.
So when your turbo is on full boost and plenty heat inside there you will boil water inside the alloy head and -----CRACK---- you are up for big bucks.
Nearly all XJs (including 4.0L) when in perfect condition seem to have about 674 places where they leak pressure. Old one have 4,892 coolant leak points
Only real diff is that the 4.0L is cast iron and uses petrol to cool the heads when there is no coolant or pressure( just joking!)
Hi flow water pumps are for ultra large block engines.
Electric water pump will solve some problems but create others.
If you have a cooling system that really is sealed to 15psi and run around 33% glycol than your problem will be the radiator and air flow.
Note with much higher specific heat, plain water is a much better coolant (carries much more heat away per volume flow), you need the glycol for a bit extra antifreeze, anti-boil and anti-rust.
Cheers