You can do it yourself with a chambering reamer and a T-handle extension for it. A word of caution, remove the ejector prior to chambering, as it is too hard to be cut by the chambering reamer. It will require opening up by a carbide end mill or a dremel tool.
Reamers can be rented instead of purchased. You can google them.
The stumbling block is the bore size. The .50 S&W is .010" undersize for the 50-70. That's like shooting .45's in a .44 bore. Don't know anyone who can rebore and rerifle with that amount to work with. The only option would be to ream and install a liner. Bob Hoyt in Fairfield, PA. can do it, I am sure there are others.