64 Dart mirror & seat belt question

Hello early A body experts!
Can the glass on the duel-post side view mirror be replaced? Also, we have another side view mirror (for the passenger side) but the ball joint is to loose so the mirror head slips, is there a way to make the ball joint tighter?
Lastly, we need to replace the seat belts for our 64 dart convertible. Modern convertibles have shoulder belts, has anyone put modern shoulder seat belts in their convertibles? I'd love suggestions on how I can get the safest belts in the car as my kid will be driving the car. Thanks!