The stock valve springs on every Magnum engine were not adequate for the stock cam, which was a ridiculously small cam to start with. If you're changing the cam you MUST put in a new set of springs.
I'm not a cam expert so I'll let others make recommendations, but I can tell you Hughes makes some good Magnum cams (bring your wallet). They also have some great reading material on their website about the Magnums:
Hughes Engines
I bought one from Hughes because they were the only company I could find that was producing an upgraded cam for the stock Magnum EFI, but you might find other manufacturers that make a good cam for your application. Good luck!
