I am familiar with SCCA and NASA rules, but they don't pertain here. SCCA rules require the shoulder straps to be horizontal or up to 20-degrees DOWNWARD (it is illogical that they be allowed to angle upward, becasue that would potentially allow the driver to move up towards the roof, as in the event of a rollover).
I'm talking about installing a five point harness in a car with no roll bar, roll cage, or harness bar. While I am concerned about safety (and my "nutsack"), the main thing I'm after is to be held snug since the leather seats are slippery. A CG-Lock is not an option, as it only keeps the lap belts snug. In my old car, I attached the should belts to the rear shock tower brace, which was at perfect height and distance (it was a 2-seater coupe), and the anti-sub belt came around the front of the seat. That setup worked perfectly for years. All I need to know is if the hooks for the child seat are strong enough to hold a 200-lb person in the event of a crash. I suppose I can try it AND use the OEM seat belt in conjunction.