You should be able to install lap belts fairly easily. Shoulder belts will be more of a problem. If you look at the photo below, of a '77 chassis, you'll see attachment points for lap belts. Unfortunately, Porsche didn't include mounts for shoulder belts in that era and some cutting and welding of the rear bulkhead would be required to install those properly. I'm sure you can buy new belts from Porsche, if you want, or buy a set of used belts from some of the guys on this forum.
JR
