The front passenger side seat belt of my 97 SHO, ratchets and locks up with very little movement, is there any way to reduce the sensitivity of the belt mechanism?
It isn't getting pulled most of the way out first (larger sized person sitting in the passenger seat can use enough belt to get close to the end. Once close to the end the belt goes into "car seat" mode as it is supposed to. When my Wife was Pregnant she had that very issue with the passenger side belt, she could get in and sit there just fine, but even if she leaned slowly towards the glove box she would pull out enough belt to make it switch.
If this is what you are running into there are a couple of things you can do to help.
1) move the passenger seat back as that makes the length to the buckle less.
2) Adjust the shoulder point on the passenger B Pillar lower as this then wastes less belt going up and back down the B pillar.
If that isn't your case you are in the boat of getting a new seat belt assembly for that side. I think Paul Nimz ran into this particular issue and replacing the belt fixed it.