actually you can make them fit if you are willing to have some machine work done.however the rods are rather a bit too long even with very short pistons. the mains are the same size as the 351w, which is why the 400 crank is a great one to swap when building a 408 windsor.
however putting the 351m or 400 rank into a 351c basically entails cutting the mains down from 3" to 2.74(i think) to fit the 351c block.