For safety - original bench or switch to bucket?

I would think you could work out a practical shoulder belt anchor on a 4-door, somewhere on the B pillar. It might help if you looked at a 70s-era A-body sedan to see how they were attached. It is probably detailed in the factory service manual for any year 1970-76.

I totally understand your thought process because I just finished a restoration of a 1966 MGB roadster. It had non-locking low-back bucket seats and lap belts. I installed locking seats with headrests from a later model, and shoulder belts, because I intend to put a lot of miles on the car. On the other hand, I don't see any practical way to do this on my Barracuda convertible, other than swapping in modern seats with built-in shoulder belts, which I am loathe to do to a very original car.