Easiest way would be a fluted replacement cylinder if legally an option.
When you wish to have your factory supplied rebated cylinder fluted you may want to check the interior depth of the chambers before starting. If the chambers reach down to include (part of) the bolt notches (like in 1st and 2nd gen 1860s) AND you like to shoot power loads all the time you may want to reconsider the fluting project.
An issue to consider is proper hardening the newly fluted cylinder.
Initiates have different opinions regarding (safe) chamber sizes of fluted cylinders in the 1860 Army.
We recently measured quite a number of Centaure C&B revolvers ("1960 NEW MODEL ARMY). Different chamber measurements were found from as far down to about the base of the nipples to as short as to only were the rebate portion on the outside of the cylinder begins. Interestingly, early production Centaure revolvers have the deep chambers, later production pistols have short chambers. Chamber sizes were reduced in 2 steps to the final short version. It appears that the last reduction of the interior depth of the chambers by the Belgian factory coincides with the introduction of the regular New Model Army with fully fluted cylinder. For details of that study check
www.1960nma.org and browse down the page "Major Characteristics" til you reach the chapter about the chamber size.
Long Johns Wolf