Dear Octavian,
SPM does not require this reslicing, it is designed to handle any required resampling implicitly (except during statistics, where all images have to be in the same space). Also, even if your in-plane grid would stay the same when resampling to istropic, you would still want 3D interpolation.
If you happen to have an image in your preferred reference space available, you could use e.g. Realign->Reslice, enter the reference image as first and the images to be resliced after it. Then select "Images 2..n" for "Resliced images" and run it. If you don't have such a reference image, you can try the volumes toolbox available here: https://sourceforge.net/projects/spmtools/files/tbxVolumes/. It has a utility that will create a reference space for you and run the interpolation in one step.
Hope this helps
Volkmar
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