I usually take the head and neck I wish to use from the same figure, then it gets a bit hard to really explain, but strategically cut both necks off in a similar way so the neck sort of fits on the body, then glue it on, and the last thing I do is optional because I don't like the hard line it leaves behind, nor do I think paint covers it well, I sculpt around it to make it look like it was always one solid piece (depending on cuts may need to do a little reconstructing too, so I always do the last part). I also have a theory but not sure if it would work, but it's cut the neck clean through in the center, place in a stilt of some sort that extends the neck to desired length, then sculpt the neck to make it look natural. It would probably work, but I haven't tried that one myself.
