My '87 has only lap belts in the rear, and I seem to see some that have shoulder belts. With what looks like a mounting point on the roll bar, I assume they can be added.
I understand that some have used seatbelts from the same-era pick-up trucks. The fastener mates with the existing lap buckle. As to the mounting points, they are already existing on your factory roll bar, and on the side of the interior cab (lift up the seat for access). Here are some photos I have downloaded from other sites on this topic.