It is always best to test the particular springs that you have in your hand. I've seen them vary all over the place.
I have a hydraulic pressure gauge (from Jegs) that I put in my mill to determine the installed height. Is really a cool tool and I have used on many engines!
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On your engine, you'll find the full-length non-rotator type springs, retainers, etc - will be easiest to find/locate. If you have a bunch of springs around, if you get a tester than you can go through them and see if you can "make a set" that are all close to the same installed/open pressures.
You need to make a little "checking spring" - out of something light that allows you to put together an assembly for each valve, install it in the engine and measure the resulting spring installed height - then you know what to do as far as shims from there.