afni.nimh.nih.gov

It sounds like you do want to use 3dClustSim, but you first need to run 3dFWHMx on your first-level residual data. And use the acf options, not fwhm. See Andy's intro, for example: [andysbrainbook.readthedocs.io]

On the other hand, if you're just exploring your pilot data results, I recommend just opening your uncorrected stat maps in a viewer and thresholding manually.

Best,
Andrew