I was a seat and seat belt engineer for 5 years at GM.
I was the guy they came to to solve customer complaints, build problems, government inquires and LAW SUITS!!
I was witness to countless tests: component, air bag, sled crash, complete car crash, and so on.
I cannot go into the details of what can happen with an improperly belted "dummy", but rest assured, it isn't pretty.
That said, with all that I have experienced, I would NEVER EVER considering installing a shoulder belt in any car, other than what came from the OEM.
So, if any of you think that you can get a nondescript shoulder belt assembly, and pick a mounting position and technique thru the process of anal extraction, that would properly save you from mayhem at any speed, then get your resumes to every car manufacturer in the world.
You will be able to save them 10's of millions of dollars in design/test/validation let alone 3 years in development time.
Stick with a lap belt properly mounted and properly worn.