I'm also going to have this problem (challenge) with some blocks I have received in a swap. Mine have ranged so far from 12" to 13". I think I will be adding at least 2" of my feature fabric and trim them to 13 1/2 or so and work with that. They will all appear to have a frame of some sort.
I love the blocks received in our swap so far ... I think some just didn't understand the different of a finished 12" block needing to have have seam allowances left on them.
Wouldn't trade them for anything though!