What you need to do is to convert the attribute to a Field. Then edit the field to link to an object on the block at the elevation you want. Then check only the Y coordinate in the field editor dialog box as shown here: Also I included the block I use for

Attached is a simple little block to illustrate how user parameters and attributes could be used in a dynamic block. We use it for stairways one plot plans and site plans. The user inputs the elevation at the top of the stairs and then pulls the stairway

Create a simple dynamic block that you can easily flip. This video shows you how create a dynamic block using a workflow that starts the same as a regular block. In the home tab on the ribbon, in the block panel, choose Create Block In the Block Definitio