Thought via Ellis Brown. He has mentioned recently of making shoe lumps for the barrels.
It would consist of the lumps and the flat of the monoblock, but most of the monoblock would be milled away (3/4"+ radius?) and the barrels brazed to that. The entire rib would need to be remade including the extension but at least the lumps might be able to be saved.

Before looking at the above, how far can you ream them out to clean up the holes? You can still solder rifle barrels in and that would be a lot of surface area.

Good luck
Tom