Consider how many valves are open at a time. The shaft bolts aren't really seeing any more load at any given time.
More frequently, yes, but total load, about the same.
Magnums are low tension rings. Between that and EFI (reduced fuel wash) a Magnum pulls compression numbers at 300K that an LA motor stopped doing at 80k.
The Magnum heads are most certainly better flow wise. Cracking between valve seats...not so much. Define 'better'.
I've tuned literally thousands of multiple-carb engines. I'm an EFI man.
Lack of fuel wash, superior overall efficiency, self-tuning during all weather and altitudes, and spark control for every condition? But carbs are better because you can tune it with a screw driver and a huge box full of jets, needles, power valves, pump cams, and vacuum guages?
Ok. The magnum is the best engine ever.
