afni.nimh.nih.gov

I'm not sure why you need the grid to be the same as another dataset when clearly this is in a different space. Still, you can accomplish this in another step with 3dresample and specify the voxel dimensions. Furthermore, the output from this *can* be used as 3dWarp's gridset option and the original 3dWarp command executed again with that difference and no additional interpolation.