If you use an Evil LPR and a stock valve then you either need to cut down the stock spring, or just run it without a spring - sounds unlikely, but many people do it without problem. When the gun is pressurized, the air pressure keeps the valve closed, so the spring just keeps the valve stem in place when it is degassed. And it won't come out easily even then becuase the o-ring holds it reasonably tightly.
Like QP said, the Evil valve comes with a new spring, which is shorter, and a plastic spacer in case you are using it without the Respirator; the ND valve doesn't need a spring at all because it has its own internal spring.