When you take out the old carpet, put the seat belt bolts back in the floor. Then, put the new carpet over top, and you can use a knife to cut an X through the carpet for the bolts to pass through in the perfect spot. NEVER drill through loop carpet, it will wrap around the drill bit, and pull a long run in the carpet.
It really helps to install carpet when it is hot and sunny. You lay the carpet in the sun for an hour while you are taking out the seats, and it will drop down on the floor pan perfectly.
Two doors, four doors, wagons, and ragtops.