Hi Yvan,
Are you happy that the water pump is working properly? - can you see reasonable water movement through the open top of the radiator as you vary the revs? Also (and I know its an obvious one) is the fanbelt properly tensioned? The reason I ask is that you say that the overflow tank is overflowing if I understand you correctly and so its worth considering the possibility that the coolant loss is as a result of overheating, not causing it.
Does it happen both at idle and at higher engine speeds and/or road speeds?
If you were to lose that much coolant through a head gasket failure I'd expect to see steam in the exhaust. Is there any oil in the coolant or has the oil changed colour?
Let us know what you find - it'll save much time for others in the future.
Aidan
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1967 131, 1969 131
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