THB,

I reckon that the AYA #2 sidelock (with detachable locks and ejectors) would be the ideal as it is a copy of the H&H sidelock. I believe that they are bar action sidelock and, though a back action sidelock may be stronger, they would be a great donor action. Felix, who posts here, is building a .500 (?) on one.

You should end up with a lovely double.

If you could find one of these with a pistol grip stock the conversion would be much easier. If you are worried about the stock handling the recoil you could always add extended tangs.....I've seen extended top tangs for sale by one of the metalsmiths that lurk here.

Please be sure to post photos for the rest of us bystanders to enjoy.