As a suggestion for this project.. Mr Glenn Fewless has done a spectacular job with the Farquie already as well as a wonderful job on a full boat restoration of my Alex Henry.
I'm sure that he'd do that rifle justice.

I'd be leaning towards a 450 3 1/4 instead of another caliber if a chamber bushing won't clear it up. I'm not sure that this would work with a nitro cartridge though.

Or you could easily have it lined and go a touch smaller possibly the 375 Flanged Magnum?