[MagicRat]

Lap belts were an option on these cars.

It is possible the nuts for the belts were welded to every car, whether they got belts or not.

If you can, pull up the carpet and remove the rear seat, and look for the nuts.....they will look like small, flush mounted holes, and may be plugged oro covered with sealer.

If none are present, and aftermarket installation is required.

IMO go to a shop that sets up or builds race cars and performance chassis for installation, because they will have the right experience to do it right.

Since no shoulder belts were ever offered, such a shop will be required if you want shoulder belts, too.