Problem/Explanation:

Noticed slowly missing coolant loss. '04 Disco 2 started building condensation on windshield, thought it was defrost vents were stuck at first. A/C Diagnostics resulted in no error codes. Then noticed a slowly-creeping "overheat". Having recently changed radiator, done a headgasket job, ARP studs improved brass waterpump, heavy duty fan clutch, new hoses all around, GM inline 'stat conversion with Stant #45358. I wasn't sure where problem could be from. Initially I must have bled system about three times to no diferrence, having even tried letting her cool off in all sorts of diferrent angles. Explaining symptoms and work done to my stepdad, he immediately said heater core, but I was skeptical. Then one day I notice coolant on passenger side floormat. This backed his theory up. So to test it, I have since bypassed the two heater core lines from engine bay with Dayco B87618 bypass hose. It has since held an almost steady cruising 183* with a max 189* with a/c off, 187* idle. A/C on full blast at night no longer causes windshield condensation. So I am convinced it is the heater core, and at the same time relieved it may not be the cracked block/slipped liner I was beginning to fear after so much work.

Proposed fix/idea:

Replace entire heater core assembly, and hoses. Preferably with a different heater core that reduces future probability of failure by going from o-ring seal to flanged tube to hose/clamp type. P38 owners have found a '98 Audi A6 heater core to replace o-ring setup, which although I am not yet sure if P38 heater core may fit, is the basis for this idea. I'm wondering if anyone has attempted this. Let me know your thoughts, if I do find a suitable replacement, I'll make a DIY out of this.

P38 mod reference:

Heater Core Replacement ? Audi | PaulP38A.com

 

so before and/or after all that work you never pressure tested your coolant system?
and you looking for to modify your heater core so it does not have to be changed, again after 11-years of use?

 

To be honest, it has all been things I was sure I would end up doing since before owning this beast of a car, and I used the fact that I was at 130,000 and the slow overheat as an excuse to tear into the engine and replace TTY bolts with ARP studs. Heads sent off for planing, complete valve lap and seal job. While I was waiting for heads to come back, I also changed out oil pan gasket and completely rebuilt oil pump, replacing gears, timing chain and front main. Heads came back and were inspected to be within tolerance. So, my only real worry was slipped/cracked block at this point.

 

Went to Harbor Freight and got a radiator pressure test kit. Found a drip at around 11psi from the lower radiator hose where it now (after GM thermo conversion) connects to plastic "W" splitter. Corrected hose clamp tension, drip ceased. Pumped system to 16psi. About five minutes into test, dripping from upper engine outlet above alternator. Corrected hose clamp tension, drip ceased. Now going on 20 mins and holding 16psi. Thank you Drowssap for your inputs, as these certainly c̶o̶u̶l̶d̶ ̶h̶a̶v̶e̶ ̶c̶o̶n̶t̶r̶i̶b̶u̶t̶e̶d̶ DID CONTRIBUTE to my out of proportion worries. Will also invest in a cylinder compression gauge to add to my growing collection, although probably not from HF.

Heater core is currently bypassed, so it IS a shame I didn't think of pressure test before, but the interior coolant odor, condensation on windshield, and most importantly presence of coolant on passenger side have made my mind. Going forward with heater core mod/upgrade. It would only make sense after my investments to improve cooling system. Will make sure to tighten hose clamps this time.

 

One hour later, still holding 15psi. Seems ok, as some leakage is to be expected from cheap kit, plus setting sun = dropping temps outside = minimal drop in pressure.

 

Found what seems to be an easy solution. Good news is if heater core is good, and leak is only coming from o-ring, stock heater core can be reused. Below is a thread on different forum, with main difference being that I would solder around the o-ring mating surface joint exterior with plumbing solder, creating a stronger seal. Picture attached to show where to solder. Let me know your opinions.

DII Heater Core W/O dash Removeal - How I did It - Pic Heavy - DiscoWeb Message Boards

 

I glad it helped it should be part of every land rover tool kit, i usually run mine up to 18psi.
So i cant help you with the D2 heater box, i have done a few range rovers, be glad you do have one of them, there are like 10-12 hrs