If you wanna get in-depth what you'd do is make a new module_skins entry and change the model it uses for the body, legs, head etc.
Otherwise if you just wanna overwrite the native human, make sure you actually replaced man_body8. (backup though!)
Are you working with a module? If so the texture file in the texture folder must have exactly the same name (man_body

and scanning module textures must be enabled in module.ini.
Good luck!