Hi Ranjan,
Yes, these results are likely grossly underestimating the needed cluster sizes, as the commands do not provide information about the blur size. You should be estimating the ACF blur parameters in the data, and providing those estimates as part of the 3dClustSim command.
Search for 3dClustSim to see how it is used with the ACF parameters in
AFNI_data6/FT_analysis/s12.proc.FT.align .
- rick