Might be hunting up the wrong tree.
If the right brake seems to grab hard, then maybe the left brake isn't grabbing at all. Why? Initial adjustment plays a role, but not to extent described.
20-30 miles is not normally far enough to have made a difference in the shoe lining thickness. Normally - if you're coming down Mt Washington at a clip, wear will be higher. :lol:
If the shoes aren't grabbing, then the self-adjusters won't move.
On the '69 up adjusters, adjusters are actuated in reverse as the secondary shoes grab .
Based on what's posted so far, I'd see if the left wheel cylinder can be bled and if the wheel cyl pistons both move free.
Also, review Chrysler's drum brake advice to techs
http://www.imperialclub.org/Repair/Lit/Master/204/Page01.htmand '69
http://www.imperialclub.org/Repair/Lit/Master/260/Cover.htm
