a quick visual clue is to look at the block beside the distributor. the 351W has a reinforcing rib while the 302 doesn't. there is also the same reicnforcing rib at the back of the block. another visual clus is that on the 302 the lower bolts on the thermostat housing are hidden behind the water pump while they are more exposed on the 351W sue to the taller deck height.

rgds
Mike