Hey DT--
Thanks for the suggestion--I don't know how I missed looking at Pirkle's book. As you can kind of see from his diagram (which becomes immediately apparent when you open one up), the 1866 does not really have a bolt face the way 1873s and most other firearms do--with the 1866 the edge of the bolt acts as part of the bolt face, the circular striker/firing pin acts as an "inner circle" of the bolt face, and the firing pin extension/piston acts as the center of the bolt face. In an original, the striker and the extension/piston are free to move back and forth a few thousandths, and I am curious if that will allow centerfire primers to back out (and what effect that might have, if any).
Helpfully, Pirkle explains that the centerfire modification involved replacing or modifying the rimfire striker/firing pin and then adding a central firing pin to the firing pin extension/piston. What is not obvious from his description is that the firing pin extension/piston forms part of the support for the case head when fired and there consequently is no room to simply add a centerfire striker without either making it part of the piston or shortening the piston. I decided to hold off on modifying the original spare parts I acquired at least until I work out the best solution, and am working on a reproduction piston/striker to fit the original bolt. Of course everything is stalled for now, because I didn't have a tap and die set of the right size, although they are now supposedly en route.
Again, thanks for the pointer to Pirkle's book as I had completely missed it, and it at least validates some of my process.
Best--
--DJ