I suppose you could order some .461 No.2 brass from Buffalo Arms. I did and they called me yesterday to let me know that it will be on their next shipment from Bertram.
link:
http://www.buffaloarms.com/browse.cfm/4,452.html

Then a reamer could be made to match the brass, or for a period correct measurement have a reamer made from a case fired in an original gun.

I just bought a Gibbs Farq. .461 Sporter. When I get it, I could take barrel measurements for you if you would like.