Just a thought, the automotive Z24 and industrial Z24 are not identical. Try getting oem gaskets from a Nissan forklift dealer. There may be subtle differences like coolant passageways that are the source of the problem. I would get new head bolts too. Also, how is the intake manifold gasket? All the high hour Z24s that I've come across had intake manifold gasket leaks, but I don't see how that would lead to your problem.