This type of output (.nii format) can be created with the -CLUSTSIM option (instead of -Clustsim),
if you also set environment variable AFNI_TTEST_NIICSIM to "YES" -- e.g., run with the options
3dttest++ -DAFNI_TTEST_NIICSIM=YES -CLUSTSIM ....
Note that the results in this file are the t-statistics converted to z-statistics ("scores"), as described under the '-toz' option in '3dttest++ -help'. And the betas are not output either, as described in option '-nomeans'. 3dClustSim and 3dXClustSim are concerned only with thresholding.
You won't get .nii files this way with the -ETAC option, since the 3dXClustSim program (which implements ETAC) requires data in the default compressed ".sdat" format. If necessary, I could write a "sdat->nii" converter program (the opposite already exists in program 3dtoXdataset), but I'd rather not, as that kind of task seems very tedious to me these days.
