Although you don't give enough information of what you have in there already. If your drawers are 1" less than the opening, then these will replace them without building new boxes. Now, don't get me wrong, I like the newer bottom mount slides better, but since you mentioned selling, this will make the job easier. Blum and all the major brands make side mount like these, but they don't hold up as good over the long run.
even if your boxes are only 21'' it won't hurt if the slide is longer than the box so long as the cabinet part has that much length.