The car should have come from the factory with rear belts. Remove the lower seat cushion and you should see threaded mounting plates for the seat belts at the bottom of the upper cushion. The seat belts are bolted to those mounting plates.
There should be a threaded bolt hole in an anchor on both sides of the tranny hump, and one just inside of each inner qtr. panel. If it's been covered over with seam sealer and such, it's often easier to find them from underneath the car, as the anchors should be very easy to spot.