First of all, has your original block been bored? If not, then spend the money to have it checked out. Unless you're building a 900 HP blown nitrous injected monster, there's really no need for a super-duper aftermarket block! The 351C is a thinwall casting (as well as all Ford small blocks) - the biggest concern with the Cleveland is cylinder wall thickness; some blocks suffered core shift in the manufacturing process resulting in thinner than planned/standard cylinder walls.
Of course, you could always build a Dart Windsor for close to the same money. Perhaps use those Scott Cook heads to make a Frankenstein Clevor with a 351W Dart block. It all depends on what you plan to do with that engine, and how fat your wallet is.