90-93 notchback seatbelts into an 87

Well the front seatbelts won't work either. You need belts from a 1990 up front as that's the only year of the 90-93 cars that swaps with the pre-90 cars. You could sell them to anyone doing a black interior conversion in a 91-93 however.

You could make them work, but it would require cutting and welding.

As for the rear belts, same thing. You'd have to cut the bracing out of the doner car and weld it into your car.

The problem with these mods is that the seatbelts are sensitive to the angles they are mounted at. The wrong angle and they could not work at all.

Another option is to find 79-83 black lap belts for the rear and just use those