Originals are often cracked and broken off their mounting, if there at all. Reproduction seat belts do not supply a cover at all. There are two mounting tabs that clip over the steel anchor strap when introduced in 1978. From about 1980, perhaps sooner, they gained a single screw as well. My belts were assembled from 1980 parts hence the screw.
When complete, the carpet slit would be where these belts sit at an angle according to the cover and where it sits on the carpet, but hidden under it. As you can see more clearly in newer pics, the covers are angled. Perhaps the anchor hole can be located through the carpet enough to mock up where it sits on the carpet to mark in chalk where to cut, minimum size so can adjust slightly if needed.
Firebird seatbelt outer sleeve 2 260267 LH by
Ben, on Flickr
Leather seatbelt sleeves saffron tan (12) by
Ben, on Flickr
They are reproduced, the top is nice, the bottom is crude. Better than nothing. Hurry for your 4c off deal!
https://classicta.com/Seat-Belt-Anch...1-_p_6957.html