Use the blade to cut and X in the carpet directly over the hole about three quarters of an inch across. Then insert the bolt in the hole and draw it tight. The carpet acts like a thread sealer holding the bolt firm.
Depending on the year of the impala, not all of them have the seat belt plates and holes in their stock floor. My 63 Impala convertible does not have seat belt plates or holes. My 63 Sport Sedan has fronts, but nothing in the rear. My 63 Sport coupe has too much of the floor gone to know what was up front, but in back it looks like it has make shift seat belt attachments. The new full replacement floor pans have all the attachment points on them.
In 1963 front seat belts were an option (first year offered, though Volvo had them as standard equipment back in 1958). They didn't become standard with GM until 1966 (mandated by federal law in 1967).. Rear seat belts were optional in 1966 but didn't become standard until 1972 or '74 (can't recall off the top of my head).
Go underneath the car and insert new longer bolts, thread them up into the car, with the long bolts sticking up you will be able to determine where to cut the carpet. Remove long bolts once the carpet is cut and secure you seat belt mounts.