The engineers did their homework for an engine with crap (90 on the seat) for spring pressure, and makes peak torque at 3800-3900 rpm and runs out of intake manifold and head before 5500. Stock magnums are truck engines that were never designed to operate anywhere near 6500. That being said, I’ve turned my 5.9 with 550lbs over the nose and cheap Chevy 1.5 roller rockers to 6700 more than a few times in my younger days. The stock stuff does hold up pretty well to (like said above) moderate spring pressure and rpm. But get a serious spring and an aggressive lobe in there and watch for snapped rocker bolts.
