How is the bore?
If the bore is shootable, you may be better off keeping the original chambering. There is a lot of info for the 577/450 out there. If not, lining it may be a good bet.
The forend and cleaning rod/barrel rib look very Westley Richards to me.
Bouldersmith has done some resto type work for me and does a very nice job of it - you should PM him here on NE.