Charles, your 67 OTS should have a threaded should harness mounting hole further toward the center of the car on the horizontal deck area. Feel through the remains of your upholstery, or better yet, look up at the deck through the trunk. I have seen photos of retractable belts mounted using this hole, but they use a strap to drop the retractor down on the vertical part of the deck. I believe the factory did this for either late S2 and/or some S3. Part way through this link is a photo of a similar type installation. If you do a search you will find lots to read.
Tom