Upgrading to a pair of late model 90’s Mustang bucket seats is really pretty simple.
Begin by removing the old seats and remove the seat mounting rails. Then remove the mounting tracks off the new seats and replace them with the old seat tracks. When removing the tracks from the seats (new and old) you will see that the track mounting holes is almost identical on both sets of seats.
Mount the front of the tracks on the existing front holes then drill the holes to mount the other end of the tracks to the seat frame. When mounting the new tracks, you may have to bend the seat adjustment arm so it is flatter and at a slight down angle a little to clear the front of the seat.
Mount the rails to the seat with a couple of thick washers between the seat and the track at each bolt. This gives the adjustment mechanisms room to move freely without binding on the cloth edges.
Reinstall the new seat setup and tryout the new comfy seats! You may have to adjust the final position to suit your personal likes but these are the basics.
You can see this done with a set of 80 Mustang LX seats in my 70 Mustang section.