Hi there:
I'm just back from a week away, so just noticed your post.
A couple of years ago I acquired an original CM ("Canada Militia") marked .50-60 Rimfire Peabody rifle from the 1866 "Fenian Scare" purchase, and since then I've had a centerfire conversion block made up using one of the original "new, old stock" breechblocks available at very reasonable cost from Dixie Gun Works. I've only had limited opportunity so far to try it (.... "too many toys, too little time" ....

) I use shortened .50-70 cases for it.
(Note: the original cartridge in the last picture is actually a .577 Snider cartridge, included for comparison with my ".50-60 Centerfire" cartridges.)
So far, my original has functioned flawlessly, though I certainly can't say that it is a "tack driver". That is at least partly due to the very basic sights this particular model had installed. As you might imagine, operating a Peabody rifle is rather akin to firing a Martini-action rifle - i.e. lower lever to open action, push shell into chamber, raise lever to close action - but with the added step of needing to manually #### the external hammer .... Not a "speed machine" by any means, but lots of fun if you like such old single-shot cartridge rifles, as I do.
Now that I am retired, I hope to get more time to "play" with this rifle .... and other toys, of course. Let me know if you have any specific questions. The design is quite simple ....