I went thru the same thought process when I instaaled shoulder harnesses in my 70 Challenger,,,car was built and sold new without them. As I remember, the clips were close to a hundred bucks, and was somewhat uncertain where to mount them and did not wish to mess up my headliner.

Then i had a flash of rare insight

Don't put the clips in at all. Instead buckle the passenger hanging belt to the floor mounted belt and cinch it firmly. This way not flopping about while driving the car and yet if to carry passenger, was just as convenient as clips.

And of course while driving, the driver belt was cinched about me so no concern there.

Could not be more happy with outcome.