I wanted the safest possible belt system and wasn't worried about exact authenticity, so I decided to install 3 point retractable shoulder belts. They happened to be available in a deep teal green which matched my upholstery nicely. Installation was fairly easy, especially with the Kaisers's courtesy light location. I just pulled out the light, attached some fishing line to the provided threaded backing plate and dropped it down the door pillar to line up with a 1/2" hole drilled below. After a few minutes of not so polite language, it is possible to catch the bolt in the threads (even though the plate is literally hanging by a thread!) The retractors are modern and operate freely until pulled on fast. Unfortunately, no chrome buckles available. Check out the installation below.
I am concerned that the seat is designed like those in a two door car, which fold forward for rear seat access. That means in a severe wreck, the multiplied weight of the seatback will be pushing the occupants forward. And if there's anyone in the back seat...
I realize I'm overthinking this- but anyone who has ever seen this video thinks differently about old car safety:
https://www.youtube.com/watch?v=joMK1WZjP7g&feature=kp
