I don't mean to disrespect anyone's opinion on this site. I can only go on my experiances with this o-ring problem. I have taken my heads off to try and get rid of the oil leak between the cylinder and the head at least 5 different times. I have had two different shops try to eliminate the leak but no luck. I have even had my motor rebuilt by a very famous shop and it still leaks (not as bad as before). When I took my heads off the leaks where clearly the oil passages at the o-rings. I tried using o-rings with the same outside diameter, but thicker in roundness of the ring. That didn't work. Once I was getting oil in my anti-freeze. The oil is under higher pressure than the coolant. I posted on this site my dilemma before. Whatever the reason is, it IS a common problem. If I'm not mistaken, the one passage with the insert tube is the high pressure oil going up to the head and the other are the return passages, and the larger one is the coolant and a return. I agree that if the head and cylinder are truley flat, the o-rings would seal. Most applications have the o-ring in a recess on one piece and fully encapsulated on the other. If the recesses that the o-rings set in on the cylinder and heads met, fully encapsulating the o-ring, maybe they wouldn't leak as much.

du907, looking at your pics, looks like the preparation was thorough enough, have you tried to over-torque the head bolts by say 10-15%...I find this interesting as I have never had any leaks on my 907 (touch wood)

careful with that. Those cylinder studs look like the old type that were prone to breaking. The later/better ones are black.

G.

I did a bit of research on different types of o-rings and x-rings for different designs, found this site interesting, reading about metallic o-rings, if you click on 'characteristics' and scoll down, there are some drawings, one resembles the paso setup with the machined o-ring slot on the bottom and flat plate on top, these can withstand amazing pressures.
http://www.daemar.com/o_rings_metallic_orings_333.html
if you can match these with the original size and maybe slightly thicker, also there are the "self energizing" as the pressure increases the seals increase their effectiveness, these may solve the problem, there are others as well worth having a look at