This may be a slightly messier issue than you are picturing. I am not confident about warping tensor files, since the warps would not modify the rotations in the tensors themselves, and it seems like that would be important. The warp would merely move the tensors, not re-evaluate them.

Ignoring that, note that most AFNI programs do not even handle 64-bit float data. NIFTI images will be converted to 32-bit floats upon input (there is likely terminal text about that). In this case, 3dAllineate will downgrade to 32-bits, apply the warp, and write the output.

*** Do not use nifti_tool -mod_hdr to convert to 64-bit floats. ***
This conversion would just *claim* that the output was 64-bit floats, but it would not be. It is still actually 32-bits on disk, and the dimensions would no longer fit the specified data size.

To actually convert back to 64-bits, use "nifti_tool -convert2dtype", as in something like:

nifti_tool -copy_image -infiles warped.32bit.nii.gz -convert2dtype NIFTI_TYPE_FLOAT64 -prefix warped.64bit.nii.gz

Does that seem reasonable?

- rick