I've never worked on a '71-'73 mustang column specifically, but I've worked on a lot of tilt columns over the years and Ford used similar methods for the columns from the '60s through the '90s.  Looking at an exploded view of the column I'm assuming the spring you're having issues with is the one located between the two halves that tilt (upper and lower flanges) which are held together by two pins extracted via a special tool that threads into the pins and extracts them.

Interestingly, this is the same way the Fox tilt columns are done.  I recently rebuilt the one for my '89.  I made a tool to extract the pins because I couldn't find one.  The Fox columns also have a spring between the upper and lower flanges that must be compressed to allow the pin holes to align. 

There are several ways to compress that spring.  One way is to use the steering shaft by sleeving over it with something (pipe, socket, etc) and then a washer and nut you can compress the upper flange towards the lower one by tightening the nut.  If the shaft is slip fit, you may need to hold it with vice grips, etc. at the bottom to keep it from pulling up through the column instead of compressing the spring. 

Another option may be to make a small spring compressor, two pieces of metal in a V configuration with a pivot at the crux of the V, then a bolt and nut beyond the crux.  You slip the legs of the V into the coils of the spring, then tighten down the bolt & nut, and it compresses the spring.  Here's an example of what I'm talking about:

https://www.mustangsplus.com/1964-1965-1966-1967-Mustang-Door-Hinge-Spring-Compressor.html

You may also be able to use vice grips or c-clamps on either side of the column to draw the upper and lower flanges together.  You'll need to be careful not to mar anything that can be seen once assembly is completed.

I believe what I did was to place the column in a vice with one pin hole facing up, and then got the rack engaged on the first notch on the pin in the upper flange, and using that as a pivot I just muscled the other side of the upper flange towards the lower until I lined up the first pin hole and drove in the pin.  Once that was done I flipped it over and the other side was considerably easier. 

Some checking about the shift interlock and the differences between manual an automatic columns seems to suggest that you can simply remove all the automatic shifting parts and swap the section where the lever was for one that doesn't have it (manual).  It sounds as though many Ford columns used the same parts and Mustang, Comet, Maverick, etc,. columns have interchangeable parts.  I did something similar when swapping from 3 on the tree to floor shift in my '67 Bronco.  Worked fine, though it didn't have the added complexity of being a tilt column.