Is the oil clean, not showing signs of milkiness? Look inside the oil filler cap and valve cover, is it clean or is there a brown foam. Is it using coolant?
If no to these answers, it probably isn't a cracked head.
If there was absolutely zero pressure at idle, the lifters would collapse and chatter badly.
If you go ahead and change the pump, take the time to pull the main bearing caps, one by one, and check the condition of the journals and insets for wear. Do the same with the rod bearings. Make sure to torque the caps back to specs. Badly worn bearings should be obvious.
Here is a site with dozens of bearing inserts photos, showing various types of wear and their causes. Bookmark it, it is a great resource.
http://catalog.mahleclevite.com/bearing/
Clean the oil strainer. If the pan is load with crude, it could be partially plugged.