There are two types of axles, the Homokenetic, all one piece, and the Annular two piece with the Flange. I have not heard of a Flange Kit, but when you order the Annular style, the Flange is part of the new axle.
If you already have the Annular style axles and have to replace it with another Annular, then just unbolt the 6 bolts at the flange and R&R the axle and bolt it up. no need to take the cover off the Differential.
Changing a Homokenetic to an Annular, you will still have to take off the Diff cover to R&R the axles.
I haven't worked on the 116 or 126 chassis replacing those axles, just the 123`s. they bolt up the same way just a longer axle.
Here is a thread that shows the 2 different style axles in one of the posts.
http://www.peachparts.com/shopforum/diesel-discussion/266630-homo-annular-axles.html
Charlie