I have an engine block that is supposedly a 351C. It has been lying around for a few years. Is there a stamp on the block that will confirm that this is a 351C motor?

The fuel pump mounting! One bolt hole, One stud, are vertical!! NOT Horizontal.

Marlin is correct. Additionally, the oil pressure port is in the back of the block, not in an aluminum bar-extension off the front, and there is a full-height cast-iron block extension that accepts a plain flat plate for the back of the water pump. A 302/351-W uses an aluminum front block extension that mounts the water pump & fuel pump, since the block is flat Main bearings in a 351W are 3" dia while a Cleveland main bearing is 2.70" dia.

Also the water outlet/thermostat housing is on the block above the timing chain vs. the intake manifold in the 289/302/351w motors.