My '69 convertible has factory shoulder belts. As you said, they are lacking in convenience for the driver, but to use those mounting points with a 3 point belt would be inconvenient for rear seat access.
If adding belts, be advised that the shoulder segment needs to be at least a foot longer than the lap portion to reach all the way to the buckle at the tunnel. More is better here.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
