No, you can't alter the Unifit adapters. You will need a 12" toilet. You can peruse the space specs on Terry's site linked from the top of this page. Exact space requirements vary from model to model. In other words, a 12" toilet does not mean it will use exactly 12" from the wall to the center of the flange. They frequently will need less than the listed size, but never more. I think you can find several options that will work for you.