So it is overheating? I haven't heard of that as a symptom of a cracked block. Water in the oil? Oil in the water? Are there bubbles in the coolant? You said no sign of a leak - I assume you mean down the outside of the block.
If it is heating I would be more tempted to say you have plugged coolant passages in the engine or you need to have your radiator boiled out.
Sure, they crack, but it isn't so easy to accomplish.