I know they did have issues with the rear mains being off in the block, not the retainer, the block itself, it was specifically mentioned by my engine guy when I inquired about buying a new block - He said run from any MP block.

To the OP .. I would try Barton, FHO and Bob George too see IF they might have suggestions on a repair.

Barton may have some pull with MP, he was fixing crates that had problems before.(cummins era)

MP had a bad part, but Stanton should have caught it and rejected the block initially. That is where the battle between him and MP will be.

I would be furious also !