Big differece between a 351W and 351C
351W is a physically smaller motor and utilzes the same valve covers that a 289 does. 1969 Mach 1 Mustangs had the 351W in them.
351C is larger and breathes much better. The Boss 351 is based on the Cleveland motor.
Ford tip of the day
The easiest way to tell if a Boss 302 or 351 has an original Boss engine is screw in frost plugs.