"I don't know I would convert the original block--I suppose you could do it yourself if you have the skills, but you would be reducing the value of the original. If you paid someone to alter the block, that would likely cost almost as much as a replacement centerfire block--I think they run about $350 these days."
S&S Firearms, Glendale, NY, shows the conversion block on their website for $265. You do not modify the carbine, although you might have to carefully file the front or rear surface of the conversion block if it does not just fit in as it comes. You simply remove the original block and the lever (remove the lever pivot screw, which lets the lever, lower block and the rimfire block to drop down. Then you remove the screw holding the upper block to the lower one, and install the centerfire block. Unless you have to fit the new block, it should take about 15 seconds!
You should check the barrel markings to see which model you have. The M1860 carbines were made by The Spencer Repeating Arms Company and had 22" barrels (measured from the muzzle to the face of the breechblock with the action closed. The M1865 and some later were made by Burnside. They were manufactured in 1865, but were delivered too late for use in the Civil War. Burnside carbines were issued to cavalry troops after the war for use in the Indian Wars campaigns, though a good many M1860's also were. (In 1867, one company of the 7th Cavalry was issued M1860 carbines and another had the later ones. Just depended on what was on hand from Ordnance stores.)
The M1860 carbines I have examined had barrels with the six groove rifling tapering from .545 at the breech to .535 at the muzzle. They were chambered for the #56 cartridge, aka, .56-56., which used a heel bullet and outside lubed bullets. The later ones were chambered in .56-50. rimfire, with an inside-lubed bullet measuring around .515" in diameter. Spencer also produced a slight variation of the later, designated .56-52, which was simply a slightly bottlenecked version of the .56-50, as he felt the .56-50 had too much crimp. The last two cartridges are interchangeable, and, as a matter of fact, they could be fired in the .56-56 guns, although the accuracy would probably be pretty poor. The base and rim diameters on all three are basically the same. The "fun" part of making ammo for the .56-56 is modifying modern .50-70 brass and finding a bullet that fits, either a heel bullet, or, as I did, casting hollow-base bullets from Lyman .533476AX mould, for an inside lubed bullet. Although I did develop "safe" loads of smokeless powder (the modified .50-70 brass has the same volume and the bullets have the same sectional density (weight in grains divided by the square of the diameter) as .45LC, I would definitely advise sticking with black powder.
Best of luck!