Failed Rocker Arm

JRapley said

I was trying to find the lbs/in rating for STOCK valve springs on a magnum head, no such luck at the moment. The current springs in the head are rated at 412 lbs/in. The recommended kit for the cam specs springs (Lunati 73949-16) with a rating of 363 lbs/in.

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Stock spring rate is around 275 lb/in for the Magnum. Stock LA springs were more like 225-250 lb/in.

However.... The force on the rocker is analyzed (mostly) on the valve weight, cam ramp rate, and the actual spring force, not spring rate. You can have a higher spring rate but lower actual forces; it all depends on the spring free lengths and the installed spring heights. So you should be looking at the installed heights and the resulting open and closed forces for various springs to try to get a handle on the forces on the rocker. Your lower valve lift helps lower the peak force on the rocker simply due to a lower open spring force.

Having a damper helps keep down spring vibrations which lowers the instantaneous forces on the rocker. Looks like you have a damper.

Here is an article that may help you:
Valvesprings for the Magnum