You might check the spacers that go under the seat tracks, sometimes they become cracked when overtightened upon installation. If that's not the problem, you can help to level things up with some flat washers directly on the floorpan but underneath the carpet so they won't be visible. You can also leave out the spacers on the side that sits high as well.

This was not an uncommon thing even when these cars came off the line. That's one of the first things I noticed on my very first car, a 70 GTX back in 1975.