head gasket

Engine is rebuilt? Why pull both? I would remove the bad one, look at it to determine WHY that cylinder filled with water. If you can't tell for sure where the gasket is blown, I would have that cylinder head checked for cracks. Then if you still can't determine the source of the leak, check the cylinder for a crack. Magnaflux that cylinder if you can get the stuff to check it. Then determine if the the other side needs to come off. But this is way I would approach it.