The engine block casting number on a 1969 Mustang Mach 1 351W (Windsor) engine is typically found on the
driver's side (left side) of the block, near the front,
below the intake manifold. It might be partially obscured by the starter motor or other components. You may need a flashlight and possibly a wire brush to clean off any grime for better visibility.
It's *not* always perfectly easy to see, so be prepared to search carefully.
