The trick is to:
* use to3d to import your Analyze .img files into real AFNI .HEAD/.BRIK volumes.
* make sure the anatomical and functional volumes have their centers set to the SPM originator specified in the output of mayo_analyze myspm.hdr (actually, you have to multiply the components of this coordinate by the pixdim[*], if the volume isn't cubic 1mm)
* make sure your FOV is set to dim[1]*pixdim[1]
* byteswap if mentioned in the output of mayo_analyze myspm.hdr
We use AFNI to visualize SPM volumes all the time. We highly recommend it.