Bear in mind that the belt-setup cars had lightweight glass, and wear on the UDF paint from the belt most likely wasn't a concern.
That said, it would be worth mocking up to see if you're happy with that setup.
Disconnect the glass from the regulator, bolt up a seatbelt to the glass channel, & pull it up.
Might have to modify or eliminate the inner window sweep.
Then figure out a buckle mount on the door.
"Helicopter tape" or "ClearBra" on the UDF would help preserve the the paint.
Not sure what those regulators weigh, meaning if it would all be worthwhile.