Creating sub-assemblies inside the main assembly is generally good practice, and I do it often, but it's not really top-down. Now that I have that out of the way, I don't believe you need all those planes. Your brackets appear to be symmetric, so if I had modeled them I'd have had one of the three main planes going right down the center (see below). Then it would be as simple as creating distance mates using those Part planes.
If your Parts don't have a plane going down the center you can create a new plane in the Part files, using opposing faces to drive the Plane, and use those planes for the Assembly mates.
Or depending on how the files were modeled it may be simple to edit the feature to put the plane in the center, possibly by editing the feature and choosing "Midplane" instead of "Blind" for the end condition.
I went back and fixed my part files so that the standard planes were in useful positions and the assembly worked like a champ. As always, I appreciate your help!