Hey Nativeshooter--
Thanks for the input and interest. I have a Uberti 1866 bolt that I partially fitted, but it is a different design--much more like an 1873 bolt with a sold bolt face and very different firing pin design. If the conversion I'm working on ends up not being very durable, or if the problems cannot be safely solved, then I will probably try the 1873-style bolt. But I'm hopeful that I can work things out with a little more trial and error.
For the time being my project is on hold as I'm taking time off to visit the Maryland Arms Collectors Show this weekend. I would guess that the majority of items for sale at this show are civil war era, but there is a broad range of offerings from Revolutionary War flintlocks up through K98s of WWII. If you ever have a chance to go, it is pretty amazing and usually has a strong showing of original Winchesters as well as a variety of original cowboy era handguns (Colts, Remingtons, Merwin Hulberts, S&Ws, plus lots of obscure stuff). It could easily break the bank if a person took to buying things, but it only costs $10 to look all day.