Building a cast iron block isn't easy, look at MP's first efforts. Now that World is casting blocks I'd say they would be hard to beat, although the block will weigh an extra 100# because of the extra cast iron put into places that need it.
The 440Source heads were right on initial sell but they have been promising the block for two years now and there have GOT to be some teething problems. You don't just design a block and pour it right the first time. When it comes out it'll be ZOK but if you are building NOW then either use your stock block or get one of the new World/MP pieces, the Koleno is unknown to me.
R.