Help with my first engine build (5.9 Magnum)

I THINK the Mag 380 crate motor used stock valve gear aside from springs (which you covered), with its cam, which is more aggressive (.501/513", 288/292) than yours.
Correct

And note: with what was essentially a truck long block with headers, 9:1 squeeze, and a hot cam, they made about 400hp. (I suspect that 380 was the lawyer-friendly number.)
Yup. I’ve seen tests with this engine between 410-415 hp. Pretty darn good for a stock truck block. It did well as a bracket engine.