What you have works. It places it after the Shading overlays. Append places at the end and prepend places at the beginning. As far as trying to get it in a specific location. The only way to prolly do that is edit the main .py file.
import bpy
class VIEW3D_MT_menu(bpy.types.Menu):
bl_label = "Test"
def draw(self, context):
self.layout.operator("mesh.primitive_monkey_add")
def addmenu_callback(self, context):
self.layout.menu("VIEW3D_MT_menu")
def register():
bpy.utils.register_class(VIEW3D_MT_menu)
bpy.types.VIEW3D_HT_header.append(addmenu_callback)
def unregister():
bpy.types.VIEW3D_HT_header.remove(addmenu_callback)
bpy.utils.unregister_class(VIEW3D_MT_menu)
if __name__ == "__main__":
register()