Ricardo,
A 351 Windsor and Cleveland can easily be distinguished because the thermostat housing on a Cleveland is vertical, mounted in the engine block where as the Windsor thermostat housing will be on the front of the intake manifold pointed toward the radiator.
Also if I recall correctly the Windsor should have 6 bolts on the valve cover where as the Cleveland should have 8, but keep in mind if the heads have been swapped this may not be true.
Brad