Hi mate, Those crack pipes are a pesky thing, If you don't give the block a good clean where the lip is you can guarantee it will leak, a dremel or some scotch pads usually helps clean it all up. What I would have done personally would have been to fill the system up with just water before adding any coolant, ran the engine until you were happy there was no leaks, drain the water and then add your coolant. For jobs like these, id recommend a drain pan or something similar to catch any coolant and re-use as required. drain pans are around 15 quid, and worth every penny