Hello AFNI experts

I am trying to ACPC align the aseg file from Freesurfer. To do this I did manual ACPC transformation in afni to the anatomical file (brainmask), then converted this acpc-aligned volume to nii, and attempted to use the following command to apply the acpc alignment to the aseg file (without resampling/warping the data): 3dWarpDrive -shift_rotate -base brainmask_acpc.nii -prefix aseg_acpc.nii aseg.nii

However, I received the following error:
++ 3dWarpDrive: AFNI version=AFNI_2009_12_31_1431 (Jun 22 2010) [64-bit]
++ Authored by: RW Cox
** ERROR: base and input grid dimensions don't match!
** ERROR: base is 191 X 236 X 171 voxels
** FATAL ERROR: input is 256 X 256 X 256 voxels
** Program compile date = Jun 22 2010

Am I going about this the wrong way? Suggestions for how to accomplish this would be much apprecaited.

(Also, does the ACPC alignment need to be done by hand or can I use @auto_tlrc using the -rigid_equiv option for this purpose?)

Thanks in advance,
Tracy