68 Cuda P/S converting to 73 K-Member with Manual steering

The length difference is in the steering shafts. Firm feel makes alternative couplings and , stuff. Flaming River offers some of these parts too.
There are 2 different sizes to those splined input ends of the stock box couplings. The boxes are interchangable though. You could just buy a box coupling rebuid kit and build the column you need from parts on hand.