Help with rear end

[emedina11]

I currently have a 67 that is converted to 72 front disc brakes. The current rear end is a 12 bolt, not sure of the ratio. Here's the thing...the brakes in the drums do not sit all the way into the drums. I'm wondering if whom ever I bought the truck from put 5 lug axeles into a 6 lug rear end. Does this make sence to any one? I'm not familiar with rear ends so how do I identify what it is? What is the best way about fixing this problem? Thanks for you time!