Frames are basically designated by the manufacturer, and indicated by the furniture they come with.
Officially, once you put a buttstock on a frame it is then and forever will be a rifle. You can put any length barrel on a pistol without issue, and if long enough it may qualify as a rifle if you have a rifle frame. If you put a barrel shorter than 16" on a rifle there are issues.
In reality, it is unlikely that the rifle/pistol frame issue will ever come up, but better safe than sorry.
One state in particular, NY, does not allow hunting with a pistol with a barrel over 16" long.