nice try, still looks flat to me, needs a little more blending and probably a color change.

Warp to fit body contours...

try overlay on the layer style and use the warp tool to bend the cross to follow the spine !

changes are already looking better. if you have time to play.. create an extra layer ( a play layer-one you can delete if it dones not work) and exlore dodge and burn tools. this will create light and shade within your image. I would start with setting of 20% . To create light and shade look at your original image and copy the same shading as on the body. GL.

Not bad, but as CMYK said the body isnt straight, so I guess the cross wont be straight either. Try the warp tool, otherwise liquify (but be sure that you wont distort the cross too much). Good luck!

u shold do a displacement map, to make it part of the body

Nice idea. I agree with others with the thought of trying to use warp, overlay, or other techniques to make it seem more natural.