As you have found out over time your spring constant declined and it had effects on the performance. It is suggested a valve spring is a fatigue failure waiting to happen.
On your next set of springs consider the benefits of Cryogenic technology, typically valve springs exhibit a longer life after processing.
Call Paul Carter who posts here on PY:
http://www.cartercryo.com/index.html
I've had him treat my springs and we monitor them regularly with a on the head spring tester. They have gone many years without a loss of pressure.
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'70 TA / 505 cid / same engine but revised ( previous best 10.63 at 127.05 )
Old information here:
http://www.hotrod.com/articles/0712p...tiac-trans-am/
Sponsor of the world's fastest Pontiac powered Ford Fairmont (engine)
5.14 at 140 mph (1/8 mile) , true 10.5 tire, stock type suspension
https://www.youtube.com/watch?v=qDoJnIP3HgE