I don't see seams on lowpoly mesh, it looks like that in Blender viewport for example: Maya and Blender have some smoothing settings for UVs, but in Zbrush I am not sure how can it be. For example I have a highpoly and lowpoly meshes, and I do "projectALL" LP on HP or just copy+paste UVs in UVmaster zplugin if necessary. I…
Hope you have the lowpoly as body_low_0 as well... Having the _0 on the highpoly is changing its suffix altogether, and won't match the suffix of the lowpoly.
put high poly in high poly slot, put low poly and low poly slot and bake. it always bakes information between high poly to lowpoly, olny time it would be bakeing lowpoly AO to the lowpoly is if you had the LP in both slots.