It would be (very) unusual for corrosion in an L-series block to penetrate an oil passage. If corrosion is a problem, then damage to the head would occur well before it became a problem with the block. I notice they crack checked the head, but was the head checked for corrosion damage ?? A lot of info about the cause can also be gained by examining the head gasket from the original failure (probably not possible now). In particular, the transfer passages at the back of the head (cyl 6) can be problematic if coolant is neglected/not used.
The thing to do is to inspect the head for corrosion damage to the transfer passages and carefully overlay the headgasket on the head to see if there are any 'problem areas' - use the block dowels to get the alignment right.