Do you know the manufacturer and part number of the springs? Contact them or check in an online catalog.
The real world way would be to put the spring and spring pressure gauge in a bench vise. Tighten the vice until the spring pressure gains crazy fast, thats when the spring starts binding. Then measure the height of the spring. If you have or had the heads "blueprinted" this will already be measured and noted.
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93 Thunderbird 5.0 HO
89 Firebird L03
95 F150 XL
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