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My experience is that the crack occurs near the top of the block at the intersection of the head bolt threads and the oil passage - just a couple of inches down onto the block. You may be able to see it using dental mirrors and looking through the water passages to the head. It is the threaded hole next to the oil passage.
I have always suspected that the crack might be caused by interference in the threads, i.e. dirt or debris in the blind threaded hole, and an ambitious assembly. But, it could simply be a bit of core shift,drill wander or all of them.
I am going from memory that the old bulletin from AERA (automotive engine rebuilders association) advised two possible methods of repair - one was the push rod method discussed below and another was by using a long roll pin (split pin) pushed into the hole with the split aimed at the outside of the block using green Locktite as the sealer.
My experience is that both repairs can be done in place with only the removal of the head. It may work. It is certainly expedient.
I have used the pushrod method with success. The repair was carefully done with the engine in the vehicle. This may not be the perfect method but it is still in daily service 20 years later.
Edit to add; In response to Tomís post, that I have seen a piece of the oil passage broken right out along with the bottom of the blind bolt hole. This will leak big time.
I hope that the corrosion is not that significant but it is certainly possible.