RLI, I am by no means an expert here. I did buy the book about building double rifles on shotgun actions and have read it many times. In the book he suggests uesing actions that have been proof tested so that you can see what the allowable bolt thrust would be. He also goes into not useing any high pressure cartriges. The 500S&W would be a high pressure cartrige and would probably be a bad idea to do. I think that the 50-70 or 577 Snider would be great as they offer a big bore and low pressure.
Ross Seyfried did an article about his 577 Tranter revolver and was talking about a 450gn slug at 750fps. Not to get into the 45-70 DGR debate, but in a pistol a large slug in that weight/velocity range would be controllable and should be effective on thin skinned beasts. I haven't clocked the velocity on my 70cal pinfire loads as the filler messes with the chrono readings, but the 850gn slug would sure pack a huge wallop and isn't really that bad to shoot. I have a LEE 12ga 1oz slug mold that I've modified, but haven't had a chance to shoot it yet. I'll look up the ISBN on the book and post it here if anyone wants to look for a copy.
I was thinking of 50-70 as the US has a 50cal limit unless you get a special sporting cartrige exemption and that is only for rifles.

I've been busy with school and finals, but I'll check my site and see if all the pics are still there.

http://members.fortunecity.com/howda

A local gun shop got in some new Turkish double shotguns that look real interesting. They have the Greener cross bolt and are in the $800 US range. The actions are made of 1040 which isn't really confidence inspireing, but if you worked withing the limits of the steel then it might be interesting to see what could/might be made from them.