Have you tried pulling the shoe slightly toward you when trying? In other words, toward you a little then press toward pin.
Double check that bar that is under the wheel cylinder. Double double check that your E brake cable has enough slack, if it's even a little too tight you'll be fighting losing battle. When I switched rear ends in my Dart I had the same problem, only worse. I loosened up the adjuster under the floor pan and they slipped right in, then readjusted the cable. I was able to get one side together, but the opposite side wasn't having any of it until I loosened things up.
Just a few things to double check.
