Are you sure the brake cam is centered? You have to remove the cables from the cam arm, and center it so that the pin isn't pushing the pads against the drum.
Even then, they can stick, and sometimes the only way to get it off is to pry.
You unbolt the backing plate and pull the whole axle, backing plate, and drum off in once piece and press it out, but even doing that you can destroy the backing plate.
Once they get seized there is not perfect way of doing it that doesn't risk breaking parts. I had an 01 350 that was seized up and took the backing plate, drum, and axle out and put it in my press. The backing plate exploded and the drum still hadn't budged. I finally got the drum off the axle, and then had to buy another backing plate to use when I put it back together.
My suggestion is to put discs on the front, and do a brake delete on the rear since you've already messed up your backing plate.