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

The column is shorter on PS (because the steering box is taller). ALSO the couplers have a different diameter. I just re-read your post and saw that you are switching to MANUAL. You have the manual column but not the coupler? OK..NOW I got it!
You are right: The PS coupler is a smaller diameter on the clocking (gear teeth). The earlier post is correct. Some of the trucks used it as well. Any A/B/C manual box coupler will work as well. If you can't get one elsewhere I can sell you a used manual coupler for $20 including shipping (or bring one to Carlisle)