Although we are not experts in MRIcro, we did answer a similar question a couple years ago. Basically, either use the newer MRIcron software that supports NIFTI format or convert to short integers using 3dcalc and convert in AFNI. If you choose to use the MRIcron software instead, you can save datasets as NIFTI format in almost all AFNI programs easily using something like this:
3dcopy dset+orig dset.nii
or
3dAFNItoNIFTI dset+orig # this command will create a new NIFTI dataset dset.nii
or
3dcalc -a dset+orig -prefix dset.nii -expr a
[
afni.nimh.nih.gov]
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afni.nimh.nih.gov];