There are the specs for the stock valve springs from my notes, which can be hard to find:
16-VS-446 (GM 519113) Outer
Listed Specs: OD=1.375 Free=1.985 Comp=1.203
Measured: OD=1.395 Free=2.020 Solid=0.930 Wire=0.170
16-VS-445 (GM 519112) Inner
Listed Specs: OD=0.969 Free=1.765 Comp=1.116
Measured: OD=0.965 Free=1.820 Solid=0.930 Wire=0.125
These are similar in specs to the stock (maybe slightly stronger, but are available at rockauto):
Melling valve springs (available at rockauto, same specs as SBI)
Melling VS450 Outer: OD=1.390 62# @1.586, 140# @1.134 (lift=0.452, rate=173)
Melling VS310 Inner: OD=1.050 31# @1.566, 102# @1.114 (lift=0.452, rate=157)
Total: 93# @1.586, 242# @1.134, rate=330#/in
Aftermarket springs (such as COMP Cams 988-16) tend to have a higher installed height, typically starting around 1.6".
You might be able to use these with +.050 offset keepers, ideally might need to deepen spring pockets a bit.
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