Did head gasket on my brothers EL, but not familiar with 6 so much.. argh

Basically car is cranking, not starting. No spark from original coil. Replaced it with a new one and still no spark from coil. Gets voltage at the plug. I saw one spark whilst cranking but that was it. Nothing consistent.

What else should I look out for on these things? Loose plug? Bad earth?

Cheers guys.