To be happy with the extra cylinder, you will want a GOOD cowboy gunsmith to fit the cylinder to your gun. The critical fitting would be to mate the hand and the star of the cylinder.
Now the hard part. If you intend to have a dual cylinder gun you can't simply fit the hand to the star. Rather you will have to cut the geometry of the .45 acp cylinder star to match the .45 Colt cylinder that is now in the gun.
The cylinder may work without fitting, but it will be rough and clunky and will cause wear to the hand and may throw the revolver out of time when using the cylinder. To minimize the transition, you would be better served to buy your .45 acp cylinder from EMF or another Pietta dealer. All of my Uberti revolvers are quite old and I don't know if any changes have been made to the cylinder configuration over the years, but the race and cylinder notch are different on my Uberti and Pietta revolvers. However, that still shouldn't keep them from being interchangable. The notches appear to be in the same place (eyeballing not measuring).
