Auto-Blend Not Working As Desired...

I am trying to use Auto-Blend to see if I can realistically place ab muscles onto a thin figure.   I am using Photoshop 2020.

However, when I do this, it doesn't seem to blend, but the only real change I see happen, is the area inside the marching ants (the ab muscles), just becomes very bright.

After activating the selection for the abs, I use the Select>Modify> Contract and select a number of pixels in.  Then, I shift-click both the abs and the full-body layer (minus the cut-out in the stomach/chest region). Then I go to Edit> Auto-Blend> Panorama (also tried Stacked option) and both checkboxes in this window are checked.> OK.

Not sure what I am doing wrong. Please help.

Please see attached file.  I appreciate your help!  Thanks!