I think you might be able to do this in a couple ways. You could try a maximum projection across slices. It's possible through the interface to do this already for the underlay dataset (Display menu), but for the overlay, you might want to use the command line program, 3dLocalstat. You can set a neighborhood that's a block and some number of voxels thick. If you have clusters that are close together, and you want to create a separate projection for each, you would run 3dLocalstat for each cluster index in the dataset. Another way is to use suma to display the data in 3D as rendered volumes or as surfaces (IsoSurface), then the extents of the clusters and the anatomical locations become pretty clear.