If I ended up getting a longer valve to allow for more lift (not likely but I want to know), how does that affect the spring/retainer/keeper selection? Wouldn't the higher location of the keepers increase the installed height of the spring necessitating taller springs so it doesn't lose its pressure on the seat? Or is that where you machine the pockets down to allow for the extra installed height? Using a longer valve can be used to your advantage in several ways. One critical error that many people do when installing larger valves is they try to sink the seat to the same depth as the original valve seat. This kills the SSR airflow, because as the valve size increases, so should the SSR. Plus, it also increases the chances of hitting water. So in essence, a longer valve installed correctly and having the seat created as shallow as possible may not increase the installed height that much, and can kinda' balance out and allow you to use the intended springs. The flip side to that is someone using the stock length o/s valve and having to sink the seat to retain an acceptable spring height, when they should have used a longer valve to prevent the shortcomings that they have now created by not doing so.
When do you need to start worrying about there being enough lift for the keeper to be pressed far enough down to contact the valve seal? Always! Or is coil bind always going to happen before this ever will? Not always, but coil bind is very destructive, and installing the springs too close to coil bind is just as bad, and greatly reduces spring life.
Why isn't uncompressed height a specification given on springs? Isn't that what you would use to determine spring selection for a longer valve stem? Uncompressed spring height is completely irrelevant to spring selection. What the springs installed height is determines that. You can have a taller uncompressed spring have less installed seat pressure than a shorter spring. It's only by being compressed to a specified height that has any meaning.
