If your didn't have shoulder belts it may not have the mountings for them installed.
I believe the requirement was a 1/1/68 thing so not all 68's had them till after that date, I think.
If you car had the belts but forgot to locate them prior to installing a headliner you should be able to press the headliner in to feel for them. I'd be careful assuming the mountings are there unless the conscience is that all 68's had the mountings.
