Well all the B Pillar trims should come with the hole markings embedded into the plastic You use that trim piece to mark the B pillar and you can weld in your nut to the frame where the top seat belt point mounts to. I don't know if the floor portion of the seat belt is marked but it should be linear to the top one. If you can weld you can do it. As far as the doors, you have to remove all the brackets and mechanisms that come in the 89+. Maybe salvage some 88 door panels to cover the holes that are left behind. As far as the latch it should be the same. After that I don't think there is much left.
I don't have any pictures but I do own an 1988 and I can snap some pictures for you if you would like?