How much more is it worth?

First let me start off by saying you have a very beautiful and rare Dart. My original block came to the same fate in about 1984 a very cold winter in Oregon and I was young and allowed it to sit out without proper antifreeze causing the exact same cracks. Unless you're restoring your car to try to squeeze the maximum price out of it or maybe you're just one of those guys who is wanting a matching numbers car then just keep the block and move on with a era correct 340 that hasn't had major repairs. If I was buying the car I would rather have a sound replacement than a repaired original.