Dear Jeff,

For manual cluster extent threshold, you can try to use one of these softwares (though I never used them): https://www.nitrc.org/projects/cluster_c... (Rubes_cluster_correct.m) or https://www.nitrc.org/projects/peak_nii or FIVE by Aaron Schultz (http://mrtools.mgh.harvard.edu/index.php...) or https://www.nitrc.org/projects/cluster_report/ (an extension of cluster_correct).

Be warned however that Eklund et al disadvise such approach as the extent thus set is arbitrary: as long as a cluster passes the significance threshold bar, it should be presented.

However, I backtrack on what I said earlier: Scott D. Slotnick, who is also the author of the paper "Cluster Success", a reply to Eklund's "Cluster Failure" paper, developed an alternative approach to corrected cluster correction which claims that it can estimate smoothness directly without needing an SPM.mat file, so it might work for 1st-level too if you set the right parameters (the degrees of freedom being the number of BOLD volumes). You can find the scripts and more information here: https://www2.bc.edu/sd-slotnick/scripts....

Hope this helps,
Best regards,
Stephen