hmm i dont think naca ducts work well on the underbody, it's suppose to source air outside the boundary layer without disrupting it

but on the underside of the car it might interfere with the ground effects, at the least it wont be as effective as if it were located on the bodywork, at worse it might fk up ur aerodynamics

on my imaginary NSX i would get a widebody with bigger ducts that actually source air into the engine bay, and have a dedicated channel for the air to exit with carbon fibre pieces which cover up the underbody and increase downforce