What you needed to do was to lay the brake shoes in the drums before you assembled them to see that you don't have full contact with the drums. Measure the drums and see that they're worn beyond their limits. The drums may also have bad taper due to old, weak brake shoe springs and retaining hardeware. So remove the drums and inspect the rub marks on the shoes. If they're not making complete and concentric contact with the drums replace them. Back in the day I could have drums turned and shoes arced to the drums but I doubt anyone does that anymore.