I had dragging rear drums at one time also, but I found the springs were mounted incorrectly causing the drums to pull to the rear and engaging the shoes slightly.
So many comments already, but just to add my thoughts, I've done this conversion on my Fury ( Dodge Phoenix here in Australia ) and I found the PCV valve was faulty and causing it to loose vacuum when I needed it. I repaired with one from a Nissan Skyline, it may have been slightly smaller but...