351w - intake connects to upper radiator hose, vavle covers have 6 bolts
351c - block connects to upper radiator hose, valve covers have 8 botls
351m - same as 351c I can only tell them apart by reading the casting numbers by the starter. If the motor is in a cougar or a mustang, chances are it will not be a 351m unless it was transplanted by someone very foolish
351 clevland ( and modified) have big SQUARE heads and a windsor has a 302 style head , also the way to tell is the firing order it is cast in the intake and diffrent from a 351 clevland and windsor, it is difficult to tell a modified anything after 74 is not a clevland it is a 400 or 351 modified, stick to torino cougar mustang 70-73 and your probally safe. ::
also the 351 modified is wider at the intake this is due to the taller deck hight and the 351 clevland intake wont fit on a 351modified so the intake on the modified are wider.
It's easy to tell the difference between a 351C and a 351M. If the boss that the thermostat housing rises up from(and in only the T-stat mounting area)the rest of the cast in cover about 3/4 inch it's a "M". If it's flat with the area that goes across the cast timing cover it's a "C".