I'm not a pro but I believe last time I switched interior plastics from a 92 to an 88 the bolt for the seat belt on the (?b piller?) was lower on the older car, had to cut a new hole in the plastic and hide the old one.

 

I think the 90-93 seatbelt mechanism uses a slightly different mounting design inside the interior panel. It has an extra antirotation tab or something. The main plus is that it keeps slight tension on the belt, all the time. With the earlier belts, every time you lean forward in the car and then back, the belt is left loose hanging in front of you. I find this incredibly annoying. The 90-93 belts so not have this problem.

I also think that the 90-93 belt doesn't have the black plastic button for the door to push closed when the door is closed. When the door opens, this button is released, causing the belt to tighten up some. Since the 90-93 belts stay tensioned all the time, they didn't need this button. My memory of all of this is very hazy. I did switch my first 1987 LX to the 90-93 seatbelts and loved it.

 

One thing to add...

The 1990 seats belts use the same mounting tab setup as they 87-89 seat belts. So even though the are updated like the rest of the 90-93 belts, the only year that will directly bolt into an 87-89 is a 1990. 91-93 will not bolt in without modification.

 

Mounting point for the front seat belts on the pillar were moved up for the 90-93 years so like mentioned above using 90+ front seat belts.

Rear belts well 87-89 used a lap belt while the 90-93's used over the shoulder belts so there were no factory rear lap belts produced in black other then in the 81-82 foxes I believe so you'd have to either find one of those uber rare sets or purchase the 5.0 resto ones.

 

i've got 2 or 3 sets of black lap belts from the early cars for sale back from when I used to sell mustang parts....

the 90 belts will bolt up no issues as mentioned....the 91+ has a longer tab...the tab that fits into the body that keep the seatbelt chassis from moving around is longer....it should be possible to simply slot the chassis at the appropriate spot and use them...but I have not done this.