Interesting one this is..... that does make sense if the pipes are blocked. If the viscous is working properly it still will not get the water sufficiently cool.
But something that caught my eye is that you said that your EGT did not go over 370deg C !!!!.... Is your temp (EGT) probe pre turbo or post turbo?? If post turbo, then it makes a bit better sense to me as you can add around 100 to 150 deg C onto that temperature at the exhaust ports of the head. Mine ( ok it is the 4.2 but same principal applies) is before the turbo, and on a slight uphill, I quickly get the EGT's to hit between 480 and 520. My alarm is set for 630deg C, but VERY VERY seldom let it get up past 610deg C ( for short bursts ).
I would check that that reading is accurate, as you don't want to be towing uphill and working that motor into the 650 - 700deg C and its only showing 370dec C. I might be on the wrong track though as I really don't know the ZD30 motor at all.
Grant on his 4.2 battled a bit with water temps climbing, and after having a new BIGGER radiator built and fitted, his temps are way low..... almost too low as the thermostat is now working more than before to regulate the temps to optimal running temps. I also have a huge radiator in my Iron Maiden, and even if climbing a hill, with aircon on full, under load, in heavy traffic or where ever.... my temp stays at a fraction below halfway.
good luck mate... but I think you may have solved the water temp issue