ALL power steering boxes in mopar passenger cars have 16:1 ratio.
So you original 68 power steering box has the same ratio at the M-body box.
The M-body box is going to be the same reman. part number as a F-body, J-body, 73-76 A-body, 73-74 E-body, 73-79 B-body, etc etc.
As said eariler, the later 73 and up power steering boxes have a larger output shaft diameter. So you will need to covert to a 73-76 A-body pitman, center link, AND idler arm.