If you want to stick with stock springs, then you have to decode the RPO code for your car. These will be the four codes that start with 6, 7, 8 and 9, each followed by two letters. Then you go to the 44W book to decode them and get the GM part numbers. If you don't have a 44W, send me your four RPO codes and I'll decode them for you.
For example, both of my '87 GN's are:
6YX LH FRONT SPRING (PROD #14080054; SERVICE #14080054)
7YW RH FRONT SPRING (PROD #14080053; SERVICE #14080054)
8PZ LH REAR SPRING (PROD #477512; SERVICE #477512)
9SJ RH REAR SPRING (PROD #485726; SERVICE #485727)
If you don't mind going to aftermarket springs (like me), I'd suggest something like the Moog cargo springs (#CC627). These went for about $65 a pair at O'Reillys a year ago.
Good luck,
Buzz White in Houston, TX