I have ecz-c heads there stamped with 312 on the ends and I bought mummerts y284s cam and 105# springs, does anyone know how long the valve length should be from the factory, mummert said for me to ground them down 100 thousandths, the machine shop said theres no way we can go that far, theres not much there above the spring, maybe they've been ground before, how long should they be after they've been ground down? any info helps