Howdy ptaylor,
Thanks for the tips. I was able to create the mask with just one voxel, I believe, and a picture of this can be seen in the attachment to this file. However, when I go to use the 3dmaskave command, like so:

bash-3.2$ 3dmaskave -quiet -mask tmp_mask+tlrc /Users/reynaadmin/test.results/Ado_Updated_LargeRewardSmallReward_GroupHungerRegress+tlrc.BRIK[1]
++ 3dmaskave: AFNI version=AFNI_22.0.10 (Feb 15 2022) [64-bit]
+++ 1 voxels survive the mask
0

The command outputs a value of 0. I've tried several variations of the code and I just kept on getting 0, so I figured it was time to turn to the afni gods once more.

Also attached to this document is the 3dinfo -verb output for the .Brik file I am testing on. The Thr= value for the first peak cluster coordinate is around -3.579. Below is the code that I ran based on your suggestions:

1dcat o.peak_coords.txt'{0}' | 3dUndump -master /Users/reynaadmin/test.results/Ado_Updated_LargeRewardSmallReward_GroupHungerRegress+tlrc.BRIK -xyz -overwrite -prefix tmp_mask -

3dmaskave -quiet -mask tmp_mask+tlrc /Users/reynaadmin/test.results/Ado_Updated_LargeRewardSmallReward_GroupHungerRegress+tlrc.BRIK[1]

Thanks again for all the help,
-Rookie