I'm not an expert on this, especially the early/mid 60's Mopars, but I did convert my 70 from manual to power, so hopefully some of what I learned applies here. Some of this most people know, but I will repeat it anyway.
1. Manual steering shafts are longer than power steering shafts.
2. Other than shaft length, column automatic & "3 on the tree" manual collars (column shift vs. floor shift), 90% of all columns are the same for the same year/make/model with some overlap between years....most parts can be used in a conversion.
3. The steering shaft in my 70 (& maybe yours too) "telescopes" like an antennae. It has an inner shaft & an outer shaft held in place by two plastic "sheer pins" which go through both shafts. Supposedly this is so in a front end collision, the steering shaft itself (in addition to part of the column) can collapse instead of just spearing your through the chest.
On my car 4spd car, I bought a complete column-shift automatic power steering column & transplanted just the steering shaft into my floor-shift manual steering column. The p/s column had apparently been in an accident & had partially "collapsed" (I couldn't just tell by looking), so it was about 2" too short when I tried to install it. To fix this, I pulled the inner shaft out of the outer shaft (vise + vise grips/hammer) until I could see the "wear marks" line up. I also double-checked the total length based on info. I got from FBBO members here. I drilled/tapped two holes through the outer/inner steering shafts, screwed in some plastic screws I bought at the hardware store & ground off the excess plastic... voila! I had new plastic "sheer pins".
Now the good part for you. I realized later that since I was going from a longer (manual steering) shaft to a shorter (power steering) shaft, I "could" have taken a different (& cheaper) approach. I "could" have just broken my plastic shear pins, pulled out the inner shaft, cut the inner shaft with a hack saw (and/or outer shaft) down to the power steering length, made my "home-made-hardware store" shear pins & been done. I really didn't "need" to buy the other column. Also, it's at least "possible" (I'm not sure) that I could have just "telescoped down" my shaft to the correct, shorter length and not had to cut anything at all.
Bottom line is you can find the power steering column shaft length for your year & cut down your column to the correct length....done.
