Marc,
For a .405 Winchester you really need a 12 gauge frame, for all the reasons I gave above. It would also have to have three lockups and be in as good a shape as possible. The .450 is about the upper end of the pressure scale of conversion calibers that can be successfully made, and as a beginner, I would not recommend that one to start on. Get one of two under your belt before you try a .405 and for God's sake, Proof Test it thoroughly before you shoot it by hand! How about a 45-90 or even a 45-110, those would work out well and are cheaper to shoot than the British calibers. Try to keep your pressures down below 40,000 P.S.I. and you will be happy. Whatever you decide on, be sure to mark the loading data on the barrel flats. Bob