The springs on my 87T have been swapped out for Moogs in the front and an unknown brand of cargo coils in the rear. I wanted to order a set of GM stock springs and Zak Klein helped me out. He was the parts manager at his family's Buick dealership and for many years he was a main supplier to the gnttype list members, like me.
Zak took the tag numbers from a known original GN and converted them to part numbers. This is what he sold me;
Both fronts the same,
14029396 spring 7.412
Driver side rear,
485734 spr coil 7.503
Passenger side rear,
485727 spr coil 7.503
The rear passenger side is visibly shorter than the driver side due to it being a stiffer spring.
Maybe someone else can confirm these, but this is what I will be using when I rebuild my suspension.