I'll comment WESHOOT2, Smiths are to small, Taurus already makes 454 and 480 factory guns, Dan Wessons to uncommon, Anacondas are no longer made and are expensive.
Just a couple weeks ago on another board I asked if anyone know of any gunsmiths doing 454/480/50 caliber conversions on the Anaconda platform, no responses. I would think a 5 shot 480 or 454 Anaconda would be a nice revolver, guess I'm the only one.
For many years the single action Ruger was about the only platform available for the big guns to be built on, then some like Bowen started doing the big conversions on the Redhawk/SuperRedhawk. At this point Ruger has the history of working on them so that's what people use - you may have noticed that many gun people, especially those involved in custom sixguns, are very resistant to change.