I have had this issue, not with a san anderas but my GT LTS
I needed the correct spring weight for my bulk and i found that TFT Tuning had a spring weight calculator that was very handy
They may be able to help and sell you the correct weight/stroke spring
The important things to know is the legnth of shock when uncompressed and the travel of the shock without the spring fitted which gives you the stroke
this is the calculator that will tell you what spring you need.
hope this helps and makes sence.
If its late 1990's GT and suspended im interested in getting hold of it.
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