Foreign valve springs in a Magnum

If all the parts were stock magnum parts then yes you did it wrong. As LX guy and I said the stock magnum rockers are not adjustable. If you torqued it down to 21 ft. lbs. then tried to turn it 1/2 turn more that's why it broke. It sounds like what you were reading was how to adjust a chevy, not a Chrysler. Or perhaps a Dodge magnum engine with a adjustable conversion kit installed.

You also keep mentioning "setting lash" but by your description sounds like you have a hydraulic cam if you tightened it down until you couldn't turn the pushrod then tried to tighten it more. That's pre-loading the lifter, not setting lash. Setting lash is done on a solid lifter cam and is exactly opposite of pre-loading a hydraulic lifter.

If you have a hydraulic cam and do not have adjustable rockers (which again magnum rockers are not adjustable) you just torque down the rocker bolts to 20-21 ft. lbs and leave it at that.