did they use the plug and stitch method to repair cracks or did they weld it? If it was welding you may want to consider having it align bored. How did the thrust surfaces on the crankshaft look? The only reason that I may disagree with your diagnosis about the volcanic dirt causing the failure is you mentioned that the cam bearings look good. I would think that the dirt would go straight thru the crankshaft and rod bearings right up to the cam bearings so they should at least be scuffed up some if that was the case.