Many of the newer side (B-pillar) mounted seat belts (4 door cars) are side dependent, mostly because of the button on them that retracts the belt when door is open.
My ’77 does not have that “door open retract” button on it, and I suspect (not for sure) they might be interchangeable.
I can’t remember details on 2-door cars to be any help on them.
Seat belts have a small lead weight dangling down on a plastic stick that is used to make seat belts lock during sudden deceleration. Sometimes those can be repaired, once you have them off and can look at them.
The seat belt has a stay in place retractor mechanism so, you can pull belt loose a couple of inches, and belt will stay somewhat loose. Those can be fixed in many cases as well, once you have belt off of car. Usually cleaning out old (hard) grease and replacing with fresh grease (not too much!).
Most seat belt problems can be fixed by most people.
If you have a broken spring or a cut belt, then there is not much to fix those.
The hard part is getting seat belt removed, to begin with (than fixing them once removed).
With a little work, you can make belts switch sides, in many cases (on FMJ cars).
Note: a lot of the newer seat belts (made from ~2000 and newer), don’t mess with!
They have an explosive device made into most of them, which will chinch the belt upon an accident. Those auto cinchers are made to cinch 4-6” of belt In a millisecond (they are faster than an air bag going off, which is still faster than you can see pop open).
BudW