Was the block line-bored before it was built up? This one sounds like you've got a slightly oversize rear main bearing bore that doesn't get enough "crush" to hold the bearing firmly in the block. Another possibility would be forgetting to torque the rear main correctly. Is there any overlap of the main seal into the parting line area between the block and the main cap? Just a little seal material protruding into the cap-to-block mating area can give an incorrect torque reading, and allow the bearing shells to be loose in the block. Look for little annoying details on this one! I agree, there had to be some mistakes made in machining or assembly, and you need to find out exactly what happened. Don't accept a patch-up fix!
Jerry

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