For the life of me...I'm not sure what an 1878 is....
By that time...they were all cartridge.
Other than a Colt 1878 Double shotgun.
Do you mean one of the 1873 "Peacemakers" that are imported made in percussion only? Or perhaps another model such as an 1860 Army or 1851 Navy?
If it is the percussion version of the 1873 SAA- there is no conversion cylinder available to change it to cartridge. There was a fellow that did get a cylinder for one and made it into a cartridge firing handgun a while back, but it's a whole lot of trouble for little gain.