Inliners International Forums General Discussion Engines GMC SBC valve swap

I am building a GMC with a 302 “D” port 2193983 casting head. As widely recommended I am planning on installing SBC 1.94” intake and 1.6” exhaust valves +0.100 with SBC valve springs. I had read the Z/28 spring will work in this application but they are only 1.250” in diameter, too narrow for the approx. 1.440” diameter spring seat in the head. That is much more wiggle room than I am comfortable with. I chose to use PBM #3400 dual spring w/damper valve springs. The GMC installed height spec of 1.821” is close to the Z/28 spring spec of 1.850”. With a slight cleanup of the seat to 1.440” diameter the PBM springs fit in the pocket perfectly. However, the installed height measured from the pocket to the bottom of the retainer with the 5.025” long valve is about 2.030”. How can the valve closed spring height be off by 0.21”? The damper isn’t even under compression at that height. What am I missing here?