The mounting point for the OEM 3-point shoulder belt in my '69 Dart is well above my shoulders.
Granted it's primarily designed to keep me out of the steering column in the event of a front end impact
The possibility of spinal compression and related injuries is frightening. I would look for another solution if you're using stock-height low back front buckets.
The Schroth harness setup appears to be a viable alternative if you have high back buckets with a separate headrest - as long as the top of the seat back is high enough. I don't know how they may work with an integrated headrest like OEM Mopar vintage high back buckets