The shaft in your power steering column is roughly 3 inches shorter than a manual shaft. The manual and power steering boxes also have different sized input shafts. The adapter makes up the length difference and also matches the input shaft size on the manual steering box and the power steering coupler on your column. They are also available from Mancini Racing, just ask for a power to manual steering adapter. Last time I checked they were $86 plus shipping.
If your manual box is a '73 or newer it is a large sector, '72 or older it is a small sector. Makes a difference on the pitman arm and steering setup you wind up using. Assuming you are working on a '72 Dart, if the manual box is a small sector all the existing steering gear is a direct bolt up. If the box is a large sector, you will need a manual pitman arm, idler arm, and center link from a '73 or newer A body. Hope this helps.