Whitey, on my first trip to this forum I see you are at it again with a tricky project. My take on your proposal is that a 32-40 on a 28 ga. Arrieta frame is perfectly feasible if you keep the pressures at BP levels - be aware that the US ammo manufacturers some years back had hot smokeless loads for .32-40 that were right on the heels of the .32 Winchester Special and .30-30, you would not want to use the equivalent of those. I think they are still in some of the loading manuals. But IMO after you spend a lot of time/and/or/$$$ you are going to end up with a gun that is less much less valuable than if you repaired it as a shotgun. Even if you sent it back to Spain for new barrels. No matter how well the work is done the market will consider it a bit of an abortion. I have seen several shotgun conversions at gun shows in the last few years and they don't sell for anything near what has gone into them