Well...before you order the lock, it may behoove you to take the barrel out and pull the drum.
I'm just starting a project where I'm giving my .36 caliber Pedersoli Frontier (aka Cabela's Blue Ridge) a complete workover, which includes converting it from percussion to flint. I'm having it rebored by Robert Hoyt to .45 and when I pulled the barrel and removed the drum I found that the hole for the drum was drilled really low...so low it was substantially into the next flat...NOT good. Not only would that not be acceptable when installing a vent liner and potentially unsafe, but the touch hole would be way too low. So Mr Hoyt is just going to cut the barrel off at the front of the old plug and install a new plug for me. Since it will no longer have a patent breach I'll have to refit the barrel into the appropriate location, since the lock position is fixed. The barrel will need to be located a bit further back. I could create a new patent breach once the barrel comes back, but am choosing not to.
So, as you can see, it could possibly involve a bit more than just dropping in a different lock!