Hi Kara,
I think you're still not using the standard mesh surfaces in the afni_proc.py command. What is the exact afni_proc.py command you used to process the data on the surface? The -surf_spec option should be specifying one of the standard mesh spec files, not the original ones. So for example, you'll need to use
-surf_spec std.120.YOUR_SUBJECT_ID_lh.spec
instead of
-surf YOUR_SUBJECT_ID_lh.spec
You should do this for all your subjects and the results should all come out on the same grid, ready to go to 3dttest++.
So if you did not use standard mesh surfaces for the afni_proc.py, you'll need to rerun the analysis for each of the subjects then go to the group level without having to use ConvertDset.
cheers,
Ziad