I've always cut down springs to get ride height in rear, looked up the ones I had on my GN by spring diameter and height and they were 79 Mustang rear springs. GN was already lowered when I bought it.
Problem with cutting down these springs is they can possibly fall out if you are doing Dukes of Hazzards jumps. Now my GN has been this way for 14 years without an issue and the smaller diameter allows more room for big exhaust, etc than the stock springs.
I have been following BENT6's thread on this section about his total suspension rebuild which is more than most of us will do but going to try the Hotchkis F-body rear springs on my car as I want to get it one inch lower.
Here is his thread.
http://www.turbobuick.com/forums/brakes-suspensions-tires-wheels/289865-total-suspension-build.html 1903R is the part number at Jegs.