just some thoughts
i have a uberti 1858 rem "new army" 44 persusion (8" barrel)
also a uberti 1875 rem outlaw in a 357 mag cartridge (7.5")
after reading this thread, i checked the uberti site, and saw what appears to be recently offered "conversion models" direct from uberti:
hope this link works:
http://www.uberti.com/firearms/army_navy_conversion_and_open_top.php
i wouldn't personally try to convert your percussion into a cartridge model because it looks like you have to add some type of ejection rod, flip gate, etc.
it might cost more money to do it yourself, than to buy a conversion model from the factory.
if buying a conversion model, i would like the 1860 army with the cartridge config, and based on the open top look, the richards-mason historical idea.
with the rem, i find the 2 models i listed above, look very much alike and compliment each other, one in black powder, the other in 357 / 38 mag.
when i bought my 1858 rem, that was from the gun room - shooter's choice, and bought a second black powder cylinder from them, i recall it was about $140 new, then i could practice the quick change like clint eastwood did (was it the outlaw josey wales at the end ?) this is a neat option. regards