It can be done, but it's not exactly simple. I know a guy that did it and he said he'd never do it again.

Basically, you'll have to weld in the mount points for the seatbelt anchors on the pillar and the floor, and also weld in the seatbelt receivers inboard of the seats. Then to finish the conversion, you need to mount the 88 doors (complete doors, including the door panels, windows and everything) and modify the door frame to accept the 88 doors.

Very simply, not worth it. Buy an 88. Or, you could be like me and wonder why everyone cares so much about pillar mounted seatbelts.