WIChix, I lost my nerve when I got to the cam stack -- my Waterloo was the two stitch selector levers/posts next to the cam stack. I did finally get those loosened and working. However, I did prowl everywhere for a service manual, maybe I can find something for you.
Here is a
link to the service manual. There are adjustments to the cam stack area on around page 138+ and then "Removals and Replacements" of the cam stack begins on page 158.
You might also find some help on the Facebook groups "Vintage Sewing Machines" and "Vintage Slant Singer Sewing Machines." People on both lists have had a lot of experience with 401's.
Good luck!