As far as displacement goes, I'd be tempted to match the Nissan 4.8 with the Toyota 4.5 single turbo V8 diesel as in the Landcruiser 70. The diesel produces around 40 bhp less than the petrol, but produces around 20 lb-ft more. Also, since it come in the LC 70 platform, the off-roading capabilities should be quite good.
If on the other hand we're going for inline sixes, then the only answer is the 1HD-FTE. That too, is down on power as compared to the 4.8 while being slightly better in torque. One can always tweak the diesel though. 20-30% extra not being unreasonable.
If you're talking about an open "challenge" for BHP and torque, I'd advise you to get a 5.2 litre Detroit Diesel 6V53T, also available in aluminium block. 275-350 BHP and upto 770 lb-ft of torque. It'll twist any transmission into spaghetti. The cast iron version weighs a lot though, about 1700 lbs :o
Another challenger is the 6.5 V8 turbo diesel in the Hummer H1. This one is a relatively light engine though, weighing less than 300 kgs. Turbo charged, it will give 215 BHP and 440 lb-ft. Being a pushrod engine, it's apparently not very big, as it fits reltively easily into the engine bays of Discoveries, Defenders and older Range Rovers.