Mark,
You never said how much this was going to cost you, but if it is in the $300.00 and up range, I would say you would be better off buying a European gun with a third fastener on it. As an added benefit many European 12 gauge guns are lighter in weight than most older American guns. I feel that any decent gun with a third fastener like the Greener crossbolt, or a hidden fastener like the sliding Purdy bolt, is going to be better suited for your conversion, no matter what the caliber is going to be. Good luck on your conversion. Bob