Unbolt the wheel cylinder(s) from the back side of the backing plate and disconnect any other lines, brackets, etc. that you can remove. That might create enough slack to work the drum off the shoes. Some heavy duty brakes use two single-acting wheel cylinders, one for each brake shoe. The shoe hold-down springs that keep them pulled against the backing plates might have to be cut off with a die grinder, but only do that as a last resort- - - -it might be hard to find replacements.
Jerry
