Cost be damned, Novak (the adapter people) have a direct replacement aluminum radiator that is made to cool a seriously large engine in a XJ.
I don't have one, but have talked to them about it, and will be part of my stroker build.
I do have a couple of similarly set up XJ's and my three row CSF radiator out performs my three core GDIs.
My real problem isn't the heat dissipation potential of the radiator, but air flow at crawl. At this, the only real air flow is from the stock electric fan (all updated to 97+ units)
I've bit the bullet and sprang for an OEM fan clutch only to find it worked no better than the $40 Autozone one it replaced, or my other 15+ year old originals, no real howl of wind when you need it.
As I'm not real worried about a little loss of power compared to the heat I'm barely dissipating, crawling, with the 100+ temps, and 1000ft per mile climbs on my local trails, I'm trying a 15" (stock shroud size) RV fan on a solid spacer. The stock fan has a very aggressive blade pitch (hence the noise when they try to work) yet the fan clutch never really locks up, so I'm hoping for a better compromise. Remember this is all reverse rotation stuff, the RV fan is good to 10,000rpm (don't know about the stock one) and it's not the water pump, but a dummy jackshaft it rides on, so I'm not worried about aditional loads, as long as I don't burn the back of the belt with hard starts and stops.
I'll post back how it works in a couple of runs, I'm whittling the spacer right now on the lathe, don't like the die-cast pot metal ones they sell.