On almost all of the GM cars/trucks I have converted from manual to power, the pitman arm spline was larger on the PS gear and the steering shaft had to shortened. The one exception I know of is the 605 box, which has the same size spline as the manual, but I think your 72 is too early to have one of those.
I have never converted a car from PS to manual, although I remember a few of those being done back in the dark ages on cars that were to be used for race only. Today, even race cars run PS, so I doubt that you will find any kind of commercially sold conversion kit for this.
To be on the safe side, I would say look for a donor car with manual steering and get all of your parts off of that. This should be much easier than trying to piece it together, because a lot of the parts aren't available new any more.