This is a picture of DaveM/gsxr's 602 head gasket failure -

http://www.w124performance.com/images/OM602_head/gasket_failure1.jpg

How did you verify an intact head gasket with only the valve cover off? Did you run the engine and not observe compression leaking into the timing chain area? This is a common failure. Typically accompanied by a breach in the seal around the oil feel line to the head which causes an oil leak in the #1 exhaust runner.

If you don't see oil in the coolant and there isn't pressure in the upper radiator hose after the car sits for 12 hours, I wouldn't worry about head cracks. It is a #17 or later head, right?

Sixto
87 300D