If the Adjuster won't turn, cut (or Drill out) the Holding Pins on the backside before you take any drastic measures. Also carefully put a Screwdriver between the Drum and the Backing Plate and very carefully pry. Pry all the way around a little at a time, but make sure not to put too much pressure on the Backing Plate, so you will not damage it.
I had the most stubborn Drum of my life earlier this year on a '60 Impala. The Shoes would just not release, but I finally got it off with no damage to any of the important parts.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America