FourTails:
Ellis Brown's book on this subject is probably still the best reference, and you should pick up a copy if you haven't already done so. He would not entertain the idea of building nitro doubles on ANY American action. He doesn't like Brit shotgun actions either, but has a serious preference for German actions, and insists on a strong third bite like the Greener cross-bolt.

Yes, I know some nitro double rifles have been built on Winnies or Lefevers, but the .470 cartridge has a fairly large head-diameter so the back-thrust on the standing breech would be fairly high.

Probably sounds too conservative, but without a Greener crossbolt, I reckon the .32-40 is about as high as I would go with confidence! Remember, you are setting off several tons of pyrotechnics about 6 inches in front of your face: don't take this lightly!

[Sorry vapodog, posted in on top of you. We agree though...]