Ok, I'm a bloomin' idiot! The leak was caused by using the wrong gaskets! Used the two with the small exhaust crossover holes. I didn't realize that in addition to holes being smaller, so are the outside dimensions of the gasket. The edge of the gasket was right at the edge of the intake port and eventualy gave way the exhaust pressure.
The machine shop checked the manifold and it was flat. My methods apparently weren't adequate to correctly measure any warpage.
