It's a while since I torqued down a head
but iirc don't you tighten when cold, and then again immediately the engine has warmed up, because the head bolts will naturally expand a fair bit with the temperature. I don't have precise figures to hand, but iirc steel has a cte of about 13 x10e-6 per degree. If a cold head is at 20 degC, and a hot one at 800 degC on the inside, let's say the temperature increase is about 600 C degrees.
How long is the head bolt? I don't know. Let's say it's about 150mm long. That would give an increase in length of 150 x 13 x 10e-6 x 600 mm = 1.17mm.
Again, I'm mentally on the back of a fag packet here, but if the thread pitch of the head bolt is about 1.25mm (which figure I seem to recall Ladislav gave me once for a wheel bolt!) that means the head nuts/bolts would need be tightened roughly a whole turn to get the same torque when the head is hot compared to when it's cold.
If you are only torquing the head when cold, and then assuming the next time you need do it is after 500 miles, then that might explain why the gasket is not sealing.
The "after 500 miles" requirement is to allow for stretching of the head bolts. If you imagine the bolt is tightened when hot, when it cools down again it wants to go back to being 150mm long. But the 'hot' torquing is forcing it to be under the same tension as if it was only 149mm long. Where does that extra mm go? It goes into the tension being applied to the bolt by the relatively incompressible cylinder head.
Like every other material, steel has a degree of stretchability, and after 500 miles the repeated cycling of hot and cold, tension applied (cold) and then relieved (hot) will likely have caused a permanent stretch in the head bolt, hence the need to re-check it is still holding the head on tightly enough to keep the gasket sealing.
All of this is especially important when the head bolts are new. As they age, not only does re-torquing stretch them, but the heat cycling also hardens the metal, making it more brittle and less prone to further stretching.
Now you folks are, I know, very experienced with these engines, so if I am telling you how to suck eggs I apologise - just hoping to help get to the bottom of the problem.