Chuck gives good advice......if you are installing a 5 point harness, you need to read the IHRA or NHRA rules for a legal installation. A "legal" installation would require a rollbar or rollcage. The shoulder belts always attach to the main hoop cross bar.
If you plan to use the factory belts with aftermarket seats, The factory belts bolt to the floor pan on either side of the seat and on the B pillar.