The @auto_tlrc script should produce a HEAD/BRIK file pair of your transformed orig space data. To convert it to NIfTI you can use 3dcopy or 3dAFNItoNIFTI which will allow you to automate the conversion to NIFTI format from HEAD/BRIK.
for example:
@auto_tlrc -base TT_icbm452+tlrc -input anat+orig.HEAD
will produce anat_at+tlrc.{HEAD,BRIK}
to convert to NIFTI
3dcopy anat_at+tlrc. anatAsNIFTI.nii
will produce a NIFTI version of anat_at+tlrc.