Water consumption woes 3.5efi classic

Hi all, Ive had my rangey for a while now, and its been good as gold engine wise mostly so far, but driving it back from france the other weekend it started to drink coolant.
Now for a hours running it uses maybe 3L of coolant, but never from the header tank.
When I stop, the header tank ALWAYS is full of coolant, but if I take off the top bung from the radiator, I can see the level in the rad well down.
It also doesnt overheat until its down to about 1/3rd from the bottom of the rad, until its used all the coolant above, the gauge stays firmly in the middle of the range.
Now, Ive been searching round for a water leak, and I found a small drip from the waterpump shaft and upon checking it, the pump bearings were shot, so Ive replaced the pump with a brand new one, but it hasnt cured the consumption. It had marked the seating in the front cover a bit, but until the water level drops, overheating isnt a issue...

I isolated the water heater connections to the heater at the bulkhead by connecting them to each other to create a loop, but thats made no difference either.
I cant visually see any more water leaks, although my rad looks a bit rough in the middle (lost all the tiny copper vanes between the tubes where the fan draws.

Which leads me onto, the head gasket being gone. Now the engine itself is a leaker and 150K on the odometer, lots of crankcase pressure blowing oil out the seals (and all over the trans etc) , so is it even worth trying to change out the head gaskets or is this just its final way of holding up the towel and asking for another engine less polietly this time...? In the back of my mind is the thought that some of the leaks might be down to compression pressure getting into the oil and overworking the breathers and it might magically not be as tired as Im assuming. Theres no cross contamination between the water and oil systems.

In short, would I be better off trying to get hold of a lump that has less mileage and just do the head gaskets on the replacement as a precaution, or will this one be ok with some new seals and a fresh set of gaskets? Or as my wife points out, probally all the s/h v8's in my price range will be equally as worn out and tired (but close your ears, she says why not just scrap the car!!). Marital pressure is rising, especially since it also needs a new nsf cv and sphere and seal etc at the same time...

Water problems on V8 as you have said in your post water pumb leaking along shaft, but you have replaced this If no other water leaks exist then you are down to water leaking into oil or into cylinders. 3.5 V8 are especialy bad for cylinders going porus with age. so the only way to check is take head off and have a look at gasket etc and see if any water tracks visible. the other solution is there is a pour in sealant for porus blocks can't remember what its called though it is only a short term solution but confirms the worst.
However its always a difficult call. so porus cylinders new bottom end.
Cylinder head gasket usually gives gass blowing through coolent. top up radiator leave off plug and start engine look for bubbles etc when reving engine.
You also said that expansion tank is not empty, sign that Gas or air getting in somewhere, double check hoses etc then you are left with above.

I took the heads off yesterday, both gaskets had evidence of blowing, and you could see a line on one between the water jacket and the cylinder.
Two cylinders on one bank had the classic clean spots near the exaust valve and one on the other, and there was carbon between a couple of the cylinders too etc. It was a bit of a mess really. It had the nasty cheapo tin head gaskets, but a composite valley gasket (but it looks like thats been one of the sources of oil leaks anyway). I have no way of checking the mating of the heads to the block/checking for warpage either so Im just fitting new ones and hoping at this stage.
Anyway, it finishes going back together in a bit, but unfortunately Ive only got the crap tin head / valley gaskets too, so I imagine Ill be revisiting the job fairly shortly and using composite gaskets next time. Id have waited and done it this time, but Im in a houseshare while Im working a contract and the landlord seemed quite keen for it to be in one piece as quick as possible. When I get back home I can take my time and I know I have the means to check the engine out properly...