If the springs are factory original, there should be a paint code on them. This will be 1, 2, or 3 stripes in white, red, or blue. In the Factory Manual, (Big Blue Book) Chapter 32 Pages 0/3 to 0/7 there is a chart telling which thickness is indicated by the paint. Now, cleaning off the road grime to enable you to see that is a challenge but can be done. If the paint appears yellow, it started as white. However, if over the years, someone has painted the springs or replaced them, you're guessing.
If you remove the spring rubber, there will be a part number on it. Chances are the number survived even after all these years.
Good luck!