What I ended up doing on my Dart project was to go to a pick-a-part yard and find nice rear seat belts that were very soft and flexible. The originals from the Dart were just too stiff and there was a lot of difference between ones in different cars. I figured that softer and slicker would be easier to work with and less likely to scuff the door's paint. I didn't put any protection where the belt rubbed the door paint. I just didn't pull the belt across the paint. The belt was only against the paint when it was snapped. And I had put a couple extra coats of paint on the top of the door in anticipation of some potential belt wear.

The other thing I ended up doing was that I didn't attach the seat belt directly to the bottom of the glass. I used a bungy cord that attached to 2 places on the bottom of the glass and then attached the belt to that loop. That way I could have some upward pressure on the glass when up and still have enough 'give' to get the belt snapped into place. Besides, should the belt stretch some, the stretch in the cord would still take up the new slack.

I had to experiment with different bungy cords to get the right tension and length, but was no big deal. They are certainly cheap enough.

Oh, and you can order door panels from Legendary with no window crank holes, if you want to go that far.