There is a "prong" that helps you mount the seatbelts in the proper place. If you look at either set, there should be a metal tab that stick out the back of them where the seatbelt retractor mounts to the body of the car. They are there so you align the retractor in a specific way. The tabs are going to be different in the two sets of retractors, so the black ones will mount slightly skewed.
You need to measure and cut a slot into the body of the car so the new seatbelts mount in the correct way. You really need to look at the car they came out of to see where the slot is on that car, so you can cut a slot in the same exact position in yours. You need to be VERY careful when doing this because if you don't get the seatbelt mounted exactly how it is supposed to be, the belt will not lock into place during an accident.
I put 91+ front belts in my 88, and had a hell of a time getting them to sit correctly. Now they are in and work perfectly, but it took a lot of measuring and planning.
If you look at the retractors, there is a wheel and ball system that is used to make them lock up during an accident, and you will notice that as you move the retractor around sometimes it will lock and sometimes it will not. That should also help you position it correctly.
That is hard to explain, I hope I helped.