Changing the barrel any any revolver, including and in particular a K-frame S&W is not a screw-it-in, screw-it-in sort of thing, and a mistake will leave you with a bent frame. Also barrels were individually fitted to each frame, and it is improbable that a barrel removed from one gun will fit perfectly on another one. The same can be said about the cylinder, yoke, and some other parts.
Most if not all of these "kits" are made up of parts removed from a revolver, which for whatever reason had to be destroyed. To meet the legal requirements the frame was destroyed after the parts were stripped off. The good folks that sell the kits presume the buyer knows that they can't simply be assembled on to another gun or frame without extensive knowledge, experience and specialized jigs and fixtures - not to mention a machine shop.
Unfortunately buyers always ask questions after they buy the parts, not before. Doing it the other way around could same them a lot of money.