Hi Silvia,
It would be very fast to rerun the same 3dttest++ command without -ClustSim. In that case, the output would include the t-stat instead of the z-score.
And it would probably run in just a few seconds. So give it a new -prefix option and remove -ClustSim.
You might also want to use 3dbucket (for example) to extract the t-stat into a separate NIFTI dataset.
- rick