Hi-

Well, there is the program p2dsetstat that can convert a p-value to a statistic, based on your dset (AFNI stats calculation store necessary information like degrees of freedom in the header of a stat):
[afni.nimh.nih.gov]
... and its complement is inventively named dsetstat2p:
[afni.nimh.nih.gov]
Note that you want to make sure that you are using appropriate sidedness when doing this:
[onlinelibrary.wiley.com]
It sounds like you have a two-sided test-- this is what most testing in MRI studies is, though it can be difficult to do this in some (other) software.

Note also that we typically recommend using both the effect estimate part of the modeling (the beta) as well as the statistic information. In particular we suggest visualizing the beta coefficients of modeling and just thresholding the stat, with the utility of this explained here:
[pubmed.ncbi.nlm.nih.gov]
... with the mechanics of doing so in AFNI explained here:
[www.youtube.com]
We have moved toward liking the "A" and "B" button features in the GUI, explained toward the end of this video, for using the "alpha" channel and "boxing" of results to display both suprathreshold and subthreshold data together in a manageable way.

This video also works in clustering in the GUI:
[www.youtube.com]

That might be more reading and viewing than you wanted.... but hopefully it provides some useful background thoughts on data presentation and threshold considerations.

--pt