You will find that you pretty well need to cut a slot in the kick pan below the backrest of the front seat. The method that I used, was to mark out a 5/8" X 3" rectangle in the center of the kick pan, about 2" - 3" below the front seat backrest, to obtain a straight as pull as possible through the floor. Within this rectangle I marked out the intended cut lines, that would enable the folding back of the metal, rather than leaving sharp edges around the rectangular hole. Use a dremel type tool with cutting wheel to make the cuts. 3/8" rubber fuel was cut and slit into appropriate lengths to slip over the folded edges of the opening, to provide additional belt protection from fraying.
Before choosing the final drilling location for bolting, be sure you have located the seat in the best location for your comfort.
It is not neccessary for you to drill or cut through any upholstery or carpet at this point for mounting the center belts.
A hole must be drilled through the floor to fasten seat belts securely. Use a length of 1/4" X 2" X 12" long steel strap iron to bolster your belt mounting point under the body pan.
The outside seat belts for the front seat will have to be mounted with the belt bracket showing on top of the carpet, with a longer cadmium coated carriage bolt, replacing the existing body to frame mounting bolt holding the fixed end of the belts.
The procedure for the rear seats belts are fairly straight forward, other than you must use a heavy support steel strap iron under the body metal for belt mounting.
Don't depend on the existing floor pan metal to provide sufficient belt mounting without a heavy back-up bolster of heavy metal.